(Slip Opinion)              OCTOBER TERM, 2009                                       1

                                       Syllabus

         NOTE: Where it is feasible, a syllabus (headnote) will be released, as is
       being done in connection with this case, at the time the opinion is issued.
       The syllabus constitutes no part of the opinion of the Court but has been
       prepared by the Reporter of Decisions for the convenience of the reader.
       See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337.


SUPREME COURT OF THE UNITED STATES

                                       Syllabus

                 BARBER ET AL. v. THOMAS, WARDEN

CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR
                  THE NINTH CIRCUIT

      No. 09–5201. Argued March 30, 2010—Decided June 7, 2010
The federal sentencing statute at issue provides that a “prisoner . . .
  serving a term of imprisonment of more than 1 year . . . may receive
  credit toward the service of [that] sentence . . . of up to 54 days at the
  end of each year” subject to the Bureau of Prison’s (BOP) “determina
  tion . . . that, during that year, the prisoner” has behaved in an ex
  emplary fashion. 18 U. S. C. §3624(b)(1). Credit “for the last year or
  portion of a year of the term of imprisonment [is] prorated . . . .” Ibid.
  The BOP applies this statute using a methodology that awards 54
  days of credit at the end of each year the prisoner serves and sets
  those days to the side. When the difference between the time re
  maining in the sentence and the amount of accumulated credit is less
  than one year, the BOP awards a prorated amount of credit for that
  final year proportional to the awards in other years.
     Petitioners claim that the BOP’s calculation method is unlawful
  because §3624(b)(1) requires a calculation based on the length of the
  term of imprisonment imposed by the sentencing judge, not the
  length of time that the prisoner actually serves. The District Court
  rejected this challenge in each of petitioner’s cases, and the Ninth
  Circuit affirmed.
Held: Because the BOP’s method for calculating good time credit re
 flects the most natural reading of the statute, it is lawful. Pp. 5–17.
    (a) The statute’s language and purpose, taken together, support the
 BOP’s method. That method tracks §3624(b)’s language by providing
 a prisoner a maximum credit of 54 days for each full year of impris
 onment and a proportionally adjusted amount of credit for any addi
 tional time served that is less than a full year. As §3624(b) directs,
 the BOP awards the credit “at the end of each year” of imprisonment.
 Petitioners’ approach cannot be reconciled with the statute. Because
2                          BARBER v. THOMAS

                                  Syllabus

    it awards credit for the sentence imposed, regardless of how much
    time is actually served, a prisoner could receive credit for a year that
    he does not spend in prison. Moreover the calculation of credit for
    such a year would not be made “at the end of” that year. Nor could
    the BOP determine whether the prisoner had exemplary behavior
    “during that year.” This language did not find its way into the stat
    ute by accident. The differences between the prior provision (re
    pealed in 1984)—which granted the prisoner a deduction at the out
    set of his sentence, subject to forfeiture for breaking prison rules—
    and the present statute—under which “credit” is “earned” “at the end
    of” the year based on an evaluation of behavior “during that year”—
    show an intent to move from a prospective entitlement to a retrospec
    tive award. The BOP’s method also furthers the basic purpose of the
    statute. Section 3624 was part of the comprehensive Sentencing Re
    form Act of 1984, which sought to achieve both increased sentencing
    uniformity and greater honesty by “mak[ing] all sentences basically
    determinate.” Mistretta v. United States, 488 U. S. 361, 367. There
    after, the sentence the judge imposed would be the one the offender
    actually served, with a sole statutory exception for good time credits.
    Ibid. Section 3624(b) states the reason for the exception: to provide
    an incentive for prisoners to “compl[y] with institutional disciplinary
    regulations.” The exception is limited and tailored to its purpose—
    credit is earned at the end of the year after compliance with institu
    tional rules is demonstrated and thereby rewards and reinforces a
    readily identifiable period of good behavior. The BOP’s approach fur
    thers §3624’s objectives by tying the award directly to good behavior
    during the preceding year. In contrast, petitioners’ approach would
    allow a prisoner to earn credit for both the portion of his sentence
    that he served and the portion offset with earned credit, which would
    loosen the statute’s connection between good behavior and the good
    time award. Pp. 5–8.
       (b) Arguments to the contrary are unconvincing. Context indicates
    that the phrase “term of imprisonment” as used in the portion of
    §3624(b) at issue here refers to prison time actually served not, as pe
    titioners contend, to the sentence imposed by the judge. Petitioners’
    reliance on legislative history is misplaced. A U. S. Sentencing Com
    mission Supplementary Report is not helpful to them either, because
    there is no indication that the Commission, in that report or in the
    Guidelines themselves, considered or referred to the particular ques
    tion whether to base good time credit on time served or the sentence
    imposed. Nor, in light of the statute’s text, structure, history, and
    purpose, is this a case in which there is a “grievous ambiguity or un
    certainty in the statute,” Muscarello v. United States, 524 U. S. 125,
    139, permitting application of the rule of lenity. Because the BOP’s
                     Cite as: 560 U. S. ____ (2010)                   3

                               Syllabus

  calculation system applies the statute as its language is most natu
  rally read, and in accordance with the statute’s basic purpose, this
  Court need not determine the extent to which Congress has granted
  the BOP authority to interpret the statute more broadly, or differ
  ently than it has done here. Cf. Chevron U. S. A. Inc. v. Natural Re
  sources Defense Council, Inc., 467 U. S. 837, 844–845. And because
  the BOP’s approach reflects the statute’s most natural reading and is
  the most consistent with its purpose, it is also preferable to the dis
  sent’s alternative interpretation. Pp. 8–17.
Affirmed.

  BREYER, J., delivered the opinion of the Court, in which ROBERTS,
C. J., and SCALIA, THOMAS, ALITO, and SOTOMAYOR, JJ., joined. KEN-
NEDY, J., filed a dissenting opinion in which STEVENS and GINSBURG,
JJ., joined.
                       Cite as: 560 U. S. ____ (2010)                              1

                            Opinion of the Court

    NOTICE: This opinion is subject to formal revision before publication in the
    preliminary print of the United States Reports. Readers are requested to
    notify the Reporter of Decisions, Supreme Court of the United States, Wash
    ington, D. C. 20543, of any typographical or other formal errors, in order
    that corrections may be made before the preliminary print goes to press.


SUPREME COURT OF THE UNITED STATES
                                  _________________

                                  No. 09–5201
                                  _________________


  MICHAEL GARY BARBER, ET AL., PETITIONERS v.
           J. E. THOMAS, WARDEN
 ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF 

            APPEALS FOR THE NINTH CIRCUIT

                                [June 7, 2010]


   JUSTICE BREYER delivered the opinion of the Court.
   Federal sentencing law permits federal prison authori
ties to award prisoners credit against prison time as a
reward for good behavior. 18 U. S. C. §3624(b). Petition
ers, two federal prisoners, challenge the method that the
Federal Bureau of Prisons uses for calculating this “good
time credit.” We conclude that the Bureau’s method re
flects the most natural reading of the statute, and we
reject petitioners’ legal challenge.
                             I

                            A

  A federal sentencing statute provides: 

    “[A] prisoner who is serving a term of imprisonment of
    more than 1 year . . . may receive credit toward the
    service of the prisoner’s sentence, beyond the time
    served, of up to 54 days at the end of each year of the
    prisoner’s term of imprisonment, beginning at the end
    of the first year of the term . . . . [C]redit for the last
    year or portion of a year of the term of imprisonment
    shall be prorated and credited within the last six
    weeks of the sentence.” §3624(b)(1).
2                   BARBER v. THOMAS

                     Opinion of the Court

The Bureau of Prisons (BOP) applies this statute using a
methodology that petitioners in this case challenge as
unlawful. In order to explain the BOP method, we shall
use a simplified example that captures its essential ele
ments. The unsimplified calculations described by the
BOP in its policy statement, see App. 96–100, will reach
approximately the same results as, and are essentially the
mathematical equivalent of, the simplified system we
describe (there may be other ways to describe the calcula
tion as well). To the extent that there are any differences
between the methodology employed by the BOP and that
reflected in our example, they are of no consequence to the
resolution of petitioners’ challenge and are therefore not
before us. Similarly, although petitioners committed their
crimes before the current version of §3624 was enacted
and are therefore subject to a previous version that dif
fered slightly in certain details, see 18 U. S. C. §3624
(1988 ed.), the differences between the two versions are
immaterial to the questions presented by this case. The
parties refer to the current version as the relevant provi
sion of law, see Brief for Petitioners 2–3; Brief for Respon
dent 8, n. 2, and we shall do the same.
   In our example we shall imagine a prisoner who has
received a sentence of 10 years’ imprisonment. We shall
assume that his behavior throughout his confinement is
exemplary and that prison authorities will consequently
consider him to merit the maximum good time credit that
the statute will allow. And we shall ignore leap years.
   Thus, at the end of the first year (Year 1) that prisoner
would earn the statute’s maximum credit of 54 days. The
relevant official (whom we shall call the “good time calcu
lator”) would note that fact and, in effect, preliminarily
put the 54 days to the side. At the end of Year 2 the pris
oner would earn an additional 54 days of good time credit.
The good time calculator would add this 54 days to the
first 54 days, note the provisional total of 108 days, and
                  Cite as: 560 U. S. ____ (2010)            3

                      Opinion of the Court

again put the 108 days’ credit to the side. By the end of
Year 8, the prisoner would have earned a total of 432 days
of good time credit (8 years times 54 days). At that time,
the good time calculator would note that the difference
between the time remaining in the sentence (2 years, or
730 days) and the amount of accumulated good time credit
(432 days) is less than 1 year (730 minus 432 equals 298
days, which is less than 365). The 432 days of good time
credit that the prisoner has earned by the end of Year 8
are sufficient to wipe out all of the last year of the 10-year
prison term and to shorten the prisoner’s 9th year of im
prisonment by 67 days.
   Year 9 of the sentence will consequently become the
prisoner’s last year of imprisonment. Further, because the
prisoner has already earned 67 days of credit against that
year (432 days already earned minus 365 days applied to
Year 10 leaves 67 days to apply to Year 9), the prisoner
will have no more than 298 days left to serve in Year 9.
Now the good time calculator will have to work out just
how much good time the prisoner can earn, and credit
against, these remaining 298 days.
   As we said, the statute provides that “good time” for this
“last year or portion” thereof shall be “prorated.” Thus,
the good time calculator must divide the 298 days into two
parts: (1) days that the prisoner will have to serve in
prison, and (2) credit for good behavior the prisoner will
earn during the days served in Year 9. In other words, the
number of days to be served in Year 9 plus the number of
good time credit days earned will be equal to the number
of days left in the sentence, namely, 298. And to keep the
award of credit in the last year proportional to awards in
other years, the ratio of these two parts of Year 9 (i.e., the
number of good time days divided by the number of days
served) must be 54/365, the same ratio that the BOP
applies to full years served. We can use some elementary
algebra, described in the Appendix, infra, to work out the
4                   BARBER v. THOMAS

                     Opinion of the Court

rest. The result is that if the prisoner serves 260 days, he
can earn an additional 38 days of credit for good behavior.
That is to say, of the 298 days remaining in his sentence,
the prisoner will have to serve 260 days in confinement,
after which point, his sentence will be fully accounted for
(given the additional 38 days’ credit earned), and he will
be released. In sum, a prisoner subject to a 10-year
(3,650-day) sentence who earns the maximum number of
days the statute permits will serve 3,180 days in confine
ment and receive 470 days of “good time” credit, about
15% of the prison time actually served.
                               B
   In this case petitioners claim that the BOP’s calculation
method is unlawful. They say that §3624(b)(1) (2006 ed.)
requires a straightforward calculation based upon the
length of the term of imprisonment that the sentencing
judge imposes, not the length of time that the prisoner
actually serves. Thus, if a sentencing judge imposes a
prison term of 10 years (as in our example), then, in peti
tioners’ view, the statute permits a maximum good time
award of 540 days (10 years times 54 days), not the 470
days that the method described above would allow. And if
the judge imposes a prison term of 10 years and 6 months,
then the statute permits 567 days (540 days for the 10
years plus 27 days for the extra 6 months), not the 494
days that the method above would allow. According to
petitioners, the BOP’s method causes model prisoners to
lose seven days of good time credit per year of imprison
ment, and because their sentences are fairly long (one,
Michael Barber, was sentenced to 26 years and 8 months;
the other, Tahir Jihad-Black, was sentenced to 21 years
and 10 months), the difference in their cases amounts to
several months of additional prison time.
   The District Court in each of these cases rejected the
prisoner’s challenge. Civ. No. 08–226 MO (D Ore., Oct. 27,
                 Cite as: 560 U. S. ____ (2010)            5

                     Opinion of the Court

2008), App. 13; Jihad-Black v. Thomas, Civ. No. 08–227
MO (D Ore., Oct. 27, 2008), App. 25. And in each instance
the Court of Appeals affirmed the District Court. Tablada
v. Thomas, No. 07–35538 (CA9, Apr. 10, 2009), App. 11;
see also Tablada v. Thomas, 533 F. 3d 800 (CA9 2008).
Because the BOP’s administration of good time credits
affects the interests of a large number of federal prisoners,
we granted the consolidated petition for certiorari to con
sider petitioners’ challenge.
                              II
   Having now considered petitioners’ arguments, we
conclude that that we must reject their legal challenge.
The statute’s language and its purpose, taken together,
convince us that the BOP’s calculation method is lawful.
For one thing, that method tracks the language of
§3624(b). That provision says that a prisoner (serving a
sentence of imprisonment of more than a year and less
than life) “may receive credit . . . of up to 54 days at the
end of each year” subject to the “determination by the
Bureau of Prisons that, during that year, the prisoner” has
behaved in an exemplary fashion. Ibid. (emphasis added).
And it says that credit for the “last year or portion of a
year . . . shall be prorated and credited within the last six
weeks of the sentence.” Ibid. As the example in Part I
makes clear, the BOP’s interpretation provides a prisoner
entitled to a maximum annual credit with 54 days of good
time credit for each full year of imprisonment that he
serves and a proportionally adjusted amount of credit for
any additional time served that is less than a full year.
And, as §3624(b) directs, the BOP awards the credit at the
end of each year of imprisonment (except, of course, for
Year 9, which is subject to the statute’s special instruction
requiring proration and crediting during the last six weeks
of the sentence).
   We are unable similarly to reconcile petitioners’ ap
6                    BARBER v. THOMAS

                      Opinion of the Court

proach with the statute. Their system awards credit for
the sentence imposed, regardless of how much time is
actually served. Thus, a prisoner under petitioners’ sys
tem could receive 54 days of credit for Year 10 despite the
fact that he would be released after less than 81⁄2 years in
prison. The good time calculation for Year 10 would not be
made “at the end of” Year 10 (nor within the last six weeks
of a sentence ending during that year). Neither could the
BOP determine whether the prisoner had behaved in
exemplary fashion “during that year.” 18 U. S. C. §3624(b)
(emphasis added); see also White v. Scibana, 390 F. 3d
997, 1001 (CA7 2004) (“The Bureau cannot evaluate a
prisoner’s behavior and award credit for good conduct if
the prisoner is not still in prison”); cf. McGinnis v. Royster,
410 U. S. 263, 273 (1973) (“Where there is no evaluation
by state officials and little or no rehabilitative participa
tion for anyone to evaluate, there is a rational justification
for declining to give good-time credit”).
   We cannot say that this language (“at the end of,” “dur
ing that year”) found its way into the statute by accident.
Under the previous good time provision, a prisoner was
“entitled to a deduction from the term of his sentence
beginning with the day on which the sentence commences
to run.” 18 U. S. C. §4161 (1982 ed.) (repealed 1984). This
deduction, granted at the outset of a prisoner’s sentence,
was then made subject to forfeiture if the prisoner “com
mit[ted] any offense or violate[d] the rules of the institu
tion.” §4165 (repealed 1984). The present statute, §3624
(2006 ed.), in contrast, creates a system under which
“credit” is “earned” “at the end of” the year based on an
evaluation of behavior “during that year.” We agree with
the Government that “[t]he textual differences between
the two statutes reveal a purpose to move from a system of
prospective entitlement to a system of retrospective
award.” Brief for Respondent 33; see also White, supra, at
1002, n. 3.
                 Cite as: 560 U. S. ____ (2010)           7

                     Opinion of the Court

   For another thing, the BOP’s method better furthers the
statute’s basic purpose. The “good time” provision in
§3624 is part of the Sentencing Reform Act of 1984, 98
Stat. 1987, 18 U. S. C. §3551 et seq., 28 U. S. C. §§991–
998, a comprehensive law that reformed federal sentenc
ing practice and directed the newly created United States
Sentencing Commission “to devise guidelines to be used
for sentencing” in district courts, Mistretta v. United
States, 488 U. S. 361, 367 (1989). Under the previous
regime, the United States Parole Commission, “as a gen
eral rule, [could] conditionally release a prisoner any time
after he serve[d] one-third of the judicially fixed term.”
United States v. Grayson, 438 U. S. 41, 47 (1978). If, for
example, a judge imposed a prison term of 15 years, the
Parole Commission might have released the prisoner after
only 5 years. And it routinely did so. See United States
Sentencing Commission, Guidelines Manual §1A3, p. s., p.
1.2 (Oct. 1987) (USSG) (“[D]efendants often serv[ed] only
about one-third of the sentence handed down by the
court”). The result was “confusion and implicit deception.”
Ibid. With the Sentencing Reform Act, Congress sought to
achieve both increased sentencing uniformity and greater
honesty by “mak[ing] all sentences basically determinate,”
Mistretta, supra, at 367. See USSG §1A3, p. s., at 1.2
(statutory objectives included “honesty in sentencing,”
“uniformity,” and “proportionality” (emphasis deleted)).
   Thereafter, the sentence the judge imposed would be the
sentence the offender actually served, with a sole statutory
exception for good time credits. Mistretta, supra, at 367 (a
“prisoner is to be released at the completion of his sen
tence reduced only by any credit earned by good behavior
while in custody” (citing §3624(b)). The reason for this
exception is provided in §3624(b) itself: to provide an
incentive for prisoners to “compl[y] with institutional
disciplinary regulations.” The good time exception is
limited (to 54 days per year) and tailored to its purpose—
8                    BARBER v. THOMAS

                      Opinion of the Court

credit is earned at the end of the year after compliance
with institutional rules is demonstrated and thereby
rewards and reinforces a readily identifiable period of good
behavior.
   The BOP’s approach furthers the objective of §3624. It
ties the award of good time credits directly to good behav
ior during the preceding year of imprisonment. By con
trast, petitioners’ approach, insofar is it would award up to
54 days per year of time sentenced as opposed to time
served, allows a prisoner to earn credit for both the portion
of his sentence that he serves and the portion of his sen
tence that he offsets with earned good time credit. In
other words, petitioners argue that the BOP should award
good time credit not only for the days a prisoner spends in
prison and behaves appropriately, but also for days that
he will not spend in prison at all, such as Year 10 in our
example. By doing so, it loosens the statute’s connection
between good behavior and the award of good time and
transforms the nature of the exception to the basic sen
tence-imposed-is-sentence-served rule. And to that extent,
it is inconsistent with the statute’s basic purpose.
                              III 

                               A

  We are not convinced by petitioners’ several arguments
against the BOP’s methodology. First, petitioners point to
the statement in §3624(b) that a prisoner “may receive
credit . . . at the end of each year of the prisoner’s term of
imprisonment.” (Emphasis added.) The words “term of
imprisonment,” they say, must refer to the years of the
term that the sentencing judge imposed (10 years in our
example), not the (less-than-10) years of the term that the
prisoner actually served once good time credits were taken
into account. After all, the very first phrase of that provi
sion makes eligible for good time credits “a prisoner who is
serving a term of imprisonment of more than 1 year other
                   Cite as: 560 U. S. ____ (2010)              9

                       Opinion of the Court

than a term of imprisonment for the duration of the pris
oner’s life.” Ibid. (emphasis added; footnote omitted). The
words “term of imprisonment” in this phrase almost cer
tainly refer to the sentence imposed, not to the time actu
ally served (otherwise prisoners sentenced to a year and a
day would become ineligible for credit as soon as they
earned it). And, as petitioners emphasize, we have recog
nized a “presumption that a given term is used to mean
the same thing throughout a statute,” Brown v. Gardner,
513 U. S. 115, 118 (1994).
  The problem for petitioners, however, is that this pre
sumption is not absolute. It yields readily to indications
that the same phrase used in different parts of the same
statute means different things, particularly where the
phrase is one that speakers can easily use in different
ways without risk of confusion. Atlantic Cleaners & Dyers,
Inc. v. United States, 286 U. S. 427, 433 (1932); General
Dynamics Land Systems, Inc. v. Cline, 540 U. S. 581, 595–
596 (2004). See, e.g., id., at 596–597 (“age” has different
meanings in the Age Discrimination in Employment Act of
1967); United States v. Cleveland Indians Baseball Co.,
532 U. S. 200, 213 (2001) (same for “ ‘wages paid’ ” in the
Internal Revenue Code); Robinson v. Shell Oil Co., 519
U. S. 337, 343–344 (1997) (same for “employee” in Title
VII of the Civil Rights Act of 1964).
  The phrase “term of imprisonment” is just such a
phrase. It can refer to the sentence that the judge im
poses, see, e.g., §3624(a) (“A prisoner shall be released” at
the end of “the prisoner’s term of imprisonment, less any
time credited” for good behavior), but it also can refer to
the time that the prisoner actually serves. Thus, §3624(d)
of the statute before us requires BOP to “furnish [a] pris
oner with . . . suitable clothing[,] . . . money, . . . and . . .
transportation” “[u]pon the release of [the] prisoner on the
expiration of the prisoner’s term of imprisonment.” (Em
phasis added.) The statute here means to assure that the
10                   BARBER v. THOMAS

                      Opinion of the Court

prisoner is provided with these necessities at the time of
his actual release from prison (sometime during Year 9 in
our example), not at the end of the term that the judge
imposed (which would be over a year later). Since the
statute uses the same phrase “term of imprisonment” in
two different ways, the presumption cannot help petition
ers here. And, for the reasons we have given, see Part II,
supra, context here indicates that the particular instance
of the phrase “term of imprisonment” at issue refers to
prison time actually served rather than the sentence
imposed by the judge.
   Second, petitioners seek to draw support from the stat
ute’s legislative history. But those who consider legisla
tive history significant cannot find that history helpful to
petitioners here. Petitioners point, for example, to a
statement in the Senate Report accompanying the Sen
tencing Reform Act, which says that the “method of calcu
lation” of good time “will be considerably less complicated
than under current law in many respects,” and that “credit
toward early release is earned at a steady and easily
determined rate that will have an obvious impact on the
prisoner’s release date.” S. Rep. No. 98–225, p. 146–147
(1983); see Brief for Petitioners 31–32. But these state
ments are consistent with the BOP’s interpretation of the
statute. Its method, as we understand it, is not particu
larly difficult to apply and it is certainly less complex than
prior law, which provided for the accumulation of two
different kinds of good time credit (general and industrial),
calculated in different manners (prospectively and retro
spectively), and awarded at different rates, depending on
the length of sentence imposed on the prisoner (5 to 10
days per month for general) or the year of employment (3
or 5 days per month for industrial). See 18 U. S. C.
§§4161, 4162 (1982 ed.).
   Petitioners also point to various statements contained in
the Act’s Conference Report and made by individual legis
                  Cite as: 560 U. S. ____ (2010)           11

                      Opinion of the Court

lators that describe good time credit as providing sentence
reductions of 15%. See Brief for Petitioners 34–36 (citing,
e.g., H. R. Conf. Rep. No. 98–1159, p. 415 (1984); 131
Cong. Rec. 488 (1985) (remarks of Rep. Hamilton)). But
there is nothing in the context of these statements to
suggest that they amounted to anything other than rough
approximations or that they were made with the present
controversy in mind. See, e.g., H. R. Conf. Rep. No. 98–
1159, at 415 (noting simply that an increase in the amount
of maximum annual credit from 36 days to 54 days “in
creases ‘good time’ that accrues from 10 percent to 15
percent”); 131 Cong. Rec. 488 (1985) (statement of Rep.
Hamilton) (“Under [pre-Sentencing Reform Act] law, about
80% of all criminals are paroled after serving one third of
their time. Now sentences will be reduced only 15% for
good behavior”). And whatever interpretive force one
attaches to legislative history, the Court normally gives
little weight to statements, such as those of the individual
legislators, made after the bill in question has become law.
See, e.g., Heintz v. Jenkins, 514 U. S. 291, 298 (1995).
   Third, petitioners rely on a statement in the United
States Sentencing Commission’s Supplementary Report on
the Initial Sentencing Guidelines and Policy Statements
issued in 1987 (hereinafter Supplementary Report). In
that Report, the Commission summarized its analysis of
recent pre-Guidelines sentencing practice, which it had
used to help draft the Guidelines. The results of the
analysis were presented in a table that permits compari
son of the likely prison-time consequences of the new
Guidelines with prison time actually served under pre-
Guidelines practice (specifically, by identifying the Guide
lines “offense level that is closest to the average time . . .
served by first-time offenders” convicted of a particular
crime, Supplementary Report 23). Because the Guidelines
“refer to sentences prior to the awarding of good time” (i.e.,
because a Guidelines sentence of, say, 30 months’ impris
12                 BARBER v. THOMAS

                    Opinion of the Court

onment does not necessarily mean that the offender will
serve the entire 30 months in prison), the Commission
adjusted the average time served “by dividing by 0.85 good
time when the term exceeded 12 months.” Ibid. This
adjustment, the Commission explained, “made sentences
in the [t]able comparable with those in the guidelines.”
Ibid.
   Pointing to this adjustment and a reference in later
editions of the Guidelines to a potential credit of “ap
proximately fifteen percent for good behavior,” see, e.g.,
USSG §1A3, p. s., at 3 (Nov. 2009), petitioners maintain
that the Commission set its Guideline ranges with the
expectation that well-behaved prisoners would receive
good time credit of up to 15% of the sentence imposed, not
15% of the time actually served. They add that, in setting
the Guidelines ranges in this way, the Commission exer
cised congressionally delegated power to interpret the
Sentencing Reform Act, see Mistretta, 488 U. S., at 371–
379 (approving Congress’ delegation of the power to prom
ulgate sentencing guidelines), and that as long as that
interpretation is reasonable, courts must defer to it. See
Chevron U. S. A. Inc. v. Natural Resources Defense Coun
cil, Inc., 467 U. S. 837, 843–844 (1984).
   Again, however, we can find no indication that the
Commission, in writing its Supplementary Report or in
the Guidelines themselves, considered or referred to the
particular question here before us, that is whether good
time credit is to be based on time served or the sentence
imposed. The Guidelines Manual itself, a more authorita
tive account of the Commission’s interpretive views than
the Supplementary Report, says nothing directly on that
subject. Moreover, with respect to comparisons between
Guidelines sentences and pre-Guidelines practice, the
original 1987 Manual cautioned that the Guidelines did
not “simply cop[y] estimates of existing practice as re
vealed by the data,” but rather “departed from the data at
                 Cite as: 560 U. S. ____ (2010)           13

                     Opinion of the Court

different points for various important reasons.” USSG
§1A3, p. s., at 1.4; see also id., §1A4(g), p. s., at 1.11
(while “Guideline sentences in many instances will ap
proximate existing [i.e., pre-Guidelines] practice,” the
Commission did “not conside[r] itself bound by existing
sentencing practice” (emphasis added)). Because the
Commission has expressed no view on the question before
us, we need not decide whether it would be entitled to
deference had it done so. If it turns out that the calcula
tion of good time credit based on prison time served rather
than the sentence imposed produces results that are more
severe than the Commission finds appropriate, the Com
mission remains free to adjust sentencing levels accord
ingly. See id., §1A2, at 1.2 (acknowledging that “the
guideline-writing process is evolutionary” and that the
Commission functions “as a permanent agency to monitor
sentencing practices in the federal courts throughout the
nation”).
   Fourth, petitioners ask us to invoke the rule of lenity
and construe §3624 (2006 ed.) in their favor, that is, in a
way that will maximize the amount of available good time
credit. We may assume for present purposes that §3624(b)
can be construed as imposing a criminal penalty. See
Bifulco v. United States, 447 U. S. 381, 387 (1980) (rule of
lenity applies to “interpretations of . . . the penalties”
imposed by “criminal prohibitions”); but see Sash v. Zenk,
428 F. 3d 132, 134 (CA2 2005) (Sotomayor, J.) (holding
that §3624(b) is not a criminal statute for the purposes of
the rule of lenity). Even so, the rule of lenity only applies
if, after considering text, structure, history, and purpose,
there remains a “grievous ambiguity or uncertainty in the
statute,” Muscarello v. United States, 524 U. S. 125, 139
(1998) (internal quotation marks omitted), such that the
Court must simply “ ‘guess as to what Congress intended.’ ”
Bifulco, supra, at 387 (quoting Ladner v. United States,
358 U. S. 169, 178 (1958)). See United States v. Hayes,
14                  BARBER v. THOMAS

                     Opinion of the Court

555 U. S. ___, ___ (2009) (slip op., at 13); United States v.
R. L. C., 503 U. S. 291, 305–306 (1992) (plurality opinion).
Having so considered the statute, we do not believe that
there remains a “grievous ambiguity or uncertainty” in the
statutory provision before us. Nor need we now simply
“guess” what the statute means.
  Finally, we note that petitioners urge us not to defer to
the BOP’s implementation of §3624(b). In our view, the
BOP’s calculation system applies that statute as its lan
guage is most naturally read, and in accordance with what
that language makes clear is its basic purpose. No one
doubts that the BOP has the legal power to implement the
statute in accordance with its language and purposes;
hence we need not determine the extent to which Congress
has granted the BOP authority to interpret the statute
more broadly, or differently than it has done here. Cf.
Chevron, supra, at 844–845.
                              B
  Acknowledging that petitioners’ arguments cannot carry
the day, the dissent has proposed a “third possibility,”
post, at 2 (opinion of KENNEDY, J.), not raised by either
party nor, to our knowledge, used elsewhere in the Crimi
nal Code. The dissent reads the statutory phrase “term of
imprisonment” to refer to “the administrative period along
which progress toward eventual freedom is marked.” Post,
at 3. It derives from this reading the following method of
calculation as applied to our 10-year example. First, “[t]he
sentence is divided into 10 365-day segments.” Ibid. At
the end of the first segment, a prisoner may receive up to
54 days of credit for good behavior. These credits immedi
ately “go toward completion of the next year” so that the
prisoner need only serve “another 311 days behind bars
before the second year of his term of imprisonment is at an
end.” Ibid. This process repeats itself until the “10th
segment,” in which a prisoner receives an unspecified
                  Cite as: 560 U. S. ____ (2010)            15

                      Opinion of the Court

“credit in a prorated amount.” Ibid. In the end, the pris
oner will have served 10 “administrative segments,” ibid.,
collectively comprising 3,117 days in prison and 533 days
of credit.
   The dissent claims “[r]eading ‘term of imprisonment’
this way is consistent with all parts of the statute.” Post,
at 4. We see at least four problems. First, the opening
sentence of §3624(a) instructs that “[a] prisoner shall be
released” upon “the expiration of the prisoner’s term of
imprisonment, less any time credited” for good behavior.
But if a prisoner’s “term of imprisonment” is the “period
that a prisoner must complete in order to earn his free
dom,” post, at 4, and it is “accounted for through a combi
nation of prison time and credits,” post, at 3, then a pris
oner should be released exactly at the end of his term of
imprisonment (without any further adjustment). Because
the dissent’s approach would require us to read words out
of the statute, or give prisoners double credit, its definition
cannot be used here.
   Second, §3624(b)(1) tells us that a prisoner receives
credit “at the end of each year” based on behavior “during
that year.” Under the dissent’s approach, however, a
prisoner may receive credit at the end of each “administra
tive segmen[t]” presumably based on his behavior during
that segment. And because an “administrative segmen[t]”
is made up of some “combination of service and credits,”
post, at 4, each one lasts less than a calendar year. We do
not see how a system in which “a prisoner may complete a
particular year of his term in less than 365 calendar days,”
ibid., and receive full good time credit for doing so, can
possibly represent the most natural reading of this statu
tory language. Nor do we know, because the BOP has not
had an opportunity to tell us, whether a system in which a
“year” lasts anywhere from 311 to 365 calendar days (and
in which the “years” of a single prisoner’s sentence may all
be of different lengths), is easily administrable. (We doubt
16                  BARBER v. THOMAS

                     Opinion of the Court

that this system will be more comprehensible to prisoners
than one, like the BOP’s, that provides credit for actual
years.)
   Third, under the dissent’s approach, credit is earned at
different rates during a single sentence. For the first
“administrative segmen[t]” in its 10-year example, the
prisoner serves 365 days and earns 54 days of credit. The
ratio of credit earned to days served is .148. For the sec
ond “administrative segmen[t],” the prisoner serves 311
days and earns 54 days of credit. This time, the ratio of
credit earned to days served is .174. (For the last “admin
istrative segmen[t],” the dissent tells us the prisoner will
receive “credit in a prorated amount,” but it does not tell
us which ratio should be used for the proration. Post, at
3.) The use of different rates finds no support in the stat
ute. The dissent objects that the statute “prescribes no
particular rate,” post, at 7, but in fact it does—54 days of
credit per year of good behavior—and it further requires
that credit for the last year be “prorated” using the same
proportion. Moreover, the dissent’s application of different
rates leads to odd results. For example, a model prisoner
sentenced on two separate 5-year terms (with a break in
between) will serve a different number of days from one
sentenced to a single 10-year term. How can this be if
both prisoners are earning 54 days of credit for each of
their 10 years in prison?
   Fourth, §3624(b)(2) provides that good time credit “shall
vest on the date the prisoner is released from custody.”
(This provision does not apply to prisoners, like petition
ers, who committed their offenses before it was amended
in 1996, but the dissent plainly intends for its approach to
apply more broadly. See post, at 9 (noting the effect on
“almost 200,000 federal prisoners”).) Yet under the dis
sent’s approach, credit appears to vest immediately. See
post, at 3 (Days of credit for the first year “go toward
completion of the next year” so that the prisoner “would
                  Cite as: 560 U. S. ____ (2010)           17

                      Opinion of the Court

need another 311 days behind bars before the second year
of his term of imprisonment is at an end”). And if it does
not, then the situation quickly becomes complicated.
What happens if, say, on the last day of the 10th “adminis
trative segmen[t]” (somewhere in the 8th calendar year), a
prisoner badly misbehaves and prison officials punish him
by taking away all of his previously earned credit? Cf. 28
CFR §541.13 (2009) (prescribing sanctions for prohibited
acts). Does the BOP retroactively adjust the duration of
all of his administrative segments to 365 days so that the
prisoner now finds himself in the middle of the 8th “ad
ministrative segmen[t]”? (Again we do not know if the
BOP would find such a system administrable, and we
doubt that this system would be more comprehensible to a
prisoner.) If so, does the prisoner have a second opportu
nity to earn credit for good behavior for the 9th “adminis
trative segmen[t]” that he had previously completed but
now must account for again? Cf. §3624(b)(1) (“Credit that
has not been earned may not later be granted”). Or, hav
ing previously awarded (and taken away) credit for that
segment, are prison authorities left without any incentive
to offer for good behavior?
  Finally, the dissent, like petitioners, invokes the rule of
lenity to support its interpretation. But, the best efforts of
the dissent notwithstanding, we still see no “grievous
ambiguity or uncertainty” that would trigger the rule’s
application. We remain convinced that the BOP’s ap
proach reflects the most natural reading of the statutory
language and the most consistent with its purpose. What
ever the merits of the dissent’s policy arguments, the
statute does not require the BOP to accept them.

 For all of these reasons, we conclude that the BOP’s
methodology is lawful. The Ninth Circuit’s judgment is

                                                    Affirmed.
18                   BARBER v. THOMAS

                     Opinion of the Court
                Appendix to opinion of the Court

                        APPENDIX

  A fuller example of the BOP’s method for calculat
  ing “credit for the last year or portion of a year of
                the term of imprisonment”
   The defendant is sentenced to 10 years’ imprisonment.
As a prisoner he exhibits exemplary behavior and is
awarded the maximum credit of 54 days at the end of each
year served in prison. At the end of Year 8, the prisoner
has 2 years remaining in his sentence and has accumu
lated 432 days of good time credit. Because the difference
between the time remaining in his sentence and the
amount of accumulated credit (i.e., 730 - 432) is less than a
year (298 days), Year 9 is the last year he will spend in
prison. (Year 10 has been completely offset by 365 of the
432 days of accumulated credit.) Further, Year 9 will be a
partial year of 298 days (the other 67 days of the year
being offset by the remainder of the accumulated credit).
   Here is where the elementary algebra comes in. We
know that x, the good time, plus y, the remaining time
served, must add up to 298. This gives us our first equa
tion: x + y = 298.
   We also know that the ratio of good time earned in the
portion of the final year to the amount of time served in
that year must equal the ratio of a full year’s good time
credit to the amount of time served in a full year. The
latter ratio is 54/365 or .148. Thus, we know that
x/y = .148, or to put it another way, x = .148y. Because we
know the value of x in terms of y, we can make a substitu
tion in our first equation to get .148y + y = 298. We then
add the two y terms together (1.148y = 298), and we solve
for y, which gives us y = 260. Now we can plug that value
into our first equation to solve for x (the good time credit).
If we subtract 260 from 298, we find that x = 38.
   The offender will have to serve 260 days in prison in
Year 9, and he will receive 38 days additional good time
                 Cite as: 560 U. S. ____ (2010)           19

                     Opinion of the Court
                Appendix to opinion of the Court

credit for that time served. The prisoner’s total good time
is 470 days (432 + 38 = 470). His total time served is 3180
days.
   As a final matter, while we have described the foregoing
as the method to calculate credit for the portion of the last
year to more transparently track the relevant statutory
language, we note that the mathematical formula can be
used to calculate the amount of maximum available credit
for an entire sentence. Using the equations supplied
above, if we divide the total number of days in a sentence
by 1.148, we get the minimum number of days that a
defendant must serve in that sentence. If we then sub
tract the number of days served from the total number of
days in the sentence, we arrive at the maximum number
of good time credit days the prisoner can earn. The stat
ute, however, awards them on a yearly basis (but for the
“last year or portion” thereof).
                 Cite as: 560 U. S. ____ (2010)            1

                    KENNEDY, J., dissenting

SUPREME COURT OF THE UNITED STATES
                         _________________

                         No. 09–5201
                         _________________


  MICHAEL GARY BARBER, ET AL., PETITIONERS v.
           J. E. THOMAS, WARDEN
 ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF 

            APPEALS FOR THE NINTH CIRCUIT

                        [June 7, 2010]


   JUSTICE KENNEDY, with whom JUSTICE STEVENS and
JUSTICE GINSBURG join, dissenting.
   The Court has interpreted a federal sentencing statute
in a manner that disadvantages almost 200,000 federal
prisoners. See Pet. for Cert. 11, and n. 2. It adopts this
reading despite the existence of an alternative interpreta
tion that is more consistent with the statute’s text. Absent
a clear congressional directive, the statute ought not to be
read as the Court reads it. For the Court’s interpreta
tion—an interpretation that in my submission is quite
incorrect—imposes tens of thousands of years of additional
prison time on federal prisoners according to a mathe
matical formula they will be unable to understand. And if
the only way to call attention to the human implications of
this case is to speak in terms of economics, then it should
be noted that the Court’s interpretation comes at a cost to
the taxpayers of untold millions of dollars. See id., at 11.
The interpretation the Court adopts, moreover, will be
devastating to the prisoners who have behaved the best
and will undermine the purpose of the statute. These
considerations, and those stated below, require this re
spectful dissent.
                            I
  The federal sentencing statute at issue here provides:
2                    BARBER v. THOMAS

                    KENNEDY, J., dissenting

    “[A] prisoner who is serving a term of imprisonment of
    more than 1 year[,] other than a term of imprisonment
    for the duration of the prisoner’s life, may receive
    credit toward the service of the prisoner’s sentence,
    beyond the time served, of up to 54 days at the end of
    each year of the prisoner’s term of imprisonment, be
    ginning at the end of the first year of the term, subject
    to determination by the Bureau of Prisons that, dur
    ing that year, the prisoner has displayed exemplary
    compliance with institutional disciplinary regula
    tions. . . . [C]redit for the last year or portion of a year
    of the term of imprisonment shall be prorated and
    credited within the last six weeks of the sentence.” 18
    U. S. C. §3624(b)(1) (emphasis added).
According to the Court, the phrase “term of imprisonment”
must mean “time actually served” the third time that it
appears in this particular subsection. But the Court gives
the phrase a different interpretation the first two times it
is used in the very same sentence. This in itself indicates
that something is quite wrong here.
   Petitioners invite the Court to read “term of imprison
ment” to mean “the sentence imposed.” This, too, seems
unworkable. And it can be acknowledged that the Court’s
rejection of this interpretation is correct.
   The choice, however, is not just between the Court’s
reading and that offered by petitioners. There is a third
possibility, one more consistent with the statute than
either of these two alternatives.
   A fair reading of the statute, and a necessary reading to
accomplish its purpose best, is to interpret the phrase
“term of imprisonment” to refer to the span of time that a
prisoner must account for in order to obtain release. The
length of the term is set at the outset by the criminal
sentence imposed. The prisoner earns release when that
term has been fully completed. Most of the term will be
                  Cite as: 560 U. S. ____ (2010)            3

                    KENNEDY, J., dissenting

satisfied through time spent behind bars. Assuming the
prisoner is well behaved, however, he may earn good time
credits along the way; and those credits may substitute for
actual prison time. Each year of the term comprises a full
365 days, which must be accounted for through a combina
tion of prison time and credits. Thus conceived, a pris
oner’s “term” is the administrative period along which
progress toward eventual freedom is marked.
   Consider the Court’s example of a prisoner subject to a
ten-year sentence. See ante, at 2–4. The sentence is
divided into ten 365-day segments. Each segment consti
tutes a year of the term. The prisoner will spend the first
365 days behind bars. In the statute’s words, he has
reached “the end of the first year of the term.” Now is the
time for credit to be awarded, and he may receive up to 54
days if sufficiently well behaved. Because he has already
completed a full year of his term, those credits go toward
completion of the next year. If, based on good behavior, he
has earned the maximum of 54 days, he would need an
other 311 days behind bars before the second year of his
term of imprisonment is at an end (because 54 + 311 =
365). If he has earned fewer than 54 days, a longer incar
ceration will be required to reach 365. Regardless, once
the prisoner reaches the end of the second year of his
term, he will again be eligible to receive good time credits.
   This process repeats itself for the third year of the term,
and so on. In the final year of his term (in this example,
the tenth segment into which his term has been divided),
the prisoner will receive credit in a prorated amount, to be
awarded “within the last six weeks of the sentence.” This
ensures that the prisoner does not reach the end of year
ten, only to find that he has just earned 54 days of credit
he no longer needs.
   The controlling rule is that each year of the prisoner’s
term—each of the ten administrative segments—
comprises 365 days that must be completed through a
4                   BARBER v. THOMAS

                    KENNEDY, J., dissenting

combination of service and credits. By combining actual
prison time with the credits he has earned, a prisoner may
complete a particular year of his term in less than 365
calendar days. As a result, credits may enable a well
behaved prisoner to complete his ten-year sentence before
ten calendar years have elapsed. For a ten-year (3,650
day) sentence, a prisoner will serve 3,117 days behind bars
if he earns a maximum of approximately 533 credits. This
is 63 more days of credit than under the Court’s reading—
more than 6 additional credit days for every year of the
sentence imposed.
   Reading “term of imprisonment” this way is consistent
with all parts of the statute. The prisoner receives his
credit “at the end of each year of [his] term of imprison
ment,” a process that “begin[s] at the end of the first year
of the term.” Credit is only awarded if the prisoner has
proven well behaved “during that year.” This interpreta
tion fulfills the “objective of §3624”—rewarding a prisoner
for exemplary conduct during the preceding year. See
ante, at 8.
   This approach also has a textual integrity that the
Court’s reading does not: It gives “term of imprisonment”
the same meaning each time it is used by the statute.
Every time it appears in §3624(b)(1), “term of imprison
ment” refers to the administrative period that a prisoner
must complete in order to earn his freedom. The Court, by
contrast, would read this phrase to mean “time actually
served” the third time it is used, but “the sentence im
posed” the first two times it is used (“ ‘a prisoner who is
serving a term of imprisonment of more than 1 year[,]
other than a term of imprisonment for the duration of the
prisoner’s life’ ”). See ante, at 8–9. The Court’s interpreta
tion thus runs afoul of the “ ‘presumption that a given
term is used to mean the same thing throughout a stat
ute.’ ” Ante, at 9 (quoting Brown v. Gardner, 513 U. S.
115, 118 (1994)). The inconsistency here is particularly
                  Cite as: 560 U. S. ____ (2010)            5

                    KENNEDY, J., dissenting

egregious because all three uses appear in the same sen
tence. See id., at 118 (“[The] presumption [is] surely at its
most vigorous when a term is repeated within a given
sentence”).
   The Court responds by noting another part of the stat
ute, a provision stating that prisoners shall receive cloth
ing, money, and transportation “[u]pon the release of [the]
prisoner on the expiration of the prisoner’s term of impris
onment.” §3624(d). A prisoner is released at the end of
his actual time behind bars, says the Court, and so “term
of imprisonment” must here refer to time actually served.
Yet release also comes at the end of a prisoner’s “term” in
the sense described above—that is, when the balance of
the sentence has been reduced to zero through a combina
tion of prison time and good time credits. Indeed, this
administrative use of the phrase fits well with the word
“expiration,” which in its most natural sense in this con
text refers to the close of a formal accounting period. See
Black’s Law Dictionary 619 (8th ed. 2004) (“A coming to
an end; esp., a formal termination on a closing date”). By
contrast, it is awkward at best to say, as the Court would
have it, that a prisoner’s actual time behind bars is some
thing that “expires.”
   The Court’s approach produces yet another oddity. The
statute requires that prorated credit be awarded for “the
last year or portion of a year of the term of imprisonment.”
One might naturally assume that the last year of a ten
year term would be year ten. That is how things work
under the approach described above, in which a ten-year
sentence is subdivided into ten administrative segments.
   But under the Court’s reading, a prisoner serving a ten
year sentence will never reach year ten of his term; year
ten simply does not exist. According to the Court, year
nine is the final year, and even year nine is not a full year:
It lasts “no more than 298 days.” Ante, at 3. If this
sounds confusing, it will be all the more so to the prisoner
6                   BARBER v. THOMAS

                    KENNEDY, J., dissenting

who has just received his sentence and turns to the statute
books to figure out when to expect his freedom.
   The Court does not even attempt to defend these flaws.
Instead, it points to four supposed defects in the approach
described above. None withstands examination.
   First, the Court notes that the statute requires the
release of a prisoner “upon ‘the expiration of the prisoner’s
term of imprisonment, less any time credited’ for good
behavior.” Ante, at 15 (quoting §3624(a)). But if “term of
imprisonment” truly refers to the entire span that a pris
oner must complete to earn his freedom—a period that
accounts both for actual time and for good time credits—
then why would the “less any time credited” language be
appropriate? The answer is that this provision—which
appears at the very beginning of the section entitled “Re
lease of a prisoner”—announces to a prisoner when release
may be expected: when the prisoner’s term expires, taking
into account credit days “as provided in subsection (b).”
§3624(a) (bold face deleted). This use of language is com
mon. A debtor who says “I will write a check for what I
owe you, less what you owe me” is simply saying “I will
pay what I owe, taking into account your debts to me.”
Perhaps the same meaning could have been conveyed
using different words, but this is hardly probative.
   Second, the Court alleges that the above approach con
flicts with the statute’s requirement that credit be
awarded “at the end of each year” based upon behavior
“during that year.” After all, if a year of the term can be
satisfied in part through credit, then it may last less than
a full calendar year. Yet the statute does not require that
credit be awarded at the end of a calendar year for good
behavior during a calendar year. What it requires is that
credit be awarded “at the end of each year of the prisoner’s
term of imprisonment” for good behavior “during that
year.” And this is precisely what the above approach does.
   Third, the Court frets that, under the approach above,
                 Cite as: 560 U. S. ____ (2010)            7

                    KENNEDY, J., dissenting

prisoners will earn credit at different rates during a single
sentence. It admonishes that “[t]he use of different rates
finds no support in the statute.” Ante, at 16. This re
sponse is telling. The statute, in fact, prescribes no par
ticular rate—and certainly no formula based on a rate—
except as embodied in one clear directive: Prisoners are
eligible to earn “up to 54 days at the end of each year of
the prisoner’s term of imprisonment.” As to that com
mand, the above approach is perfectly faithful.
   Fourth, the Court suggests that the above approach
causes credit to vest immediately, contrary to the statute.
Again, this is not true. As per the statute, credit only
vests “on the date the prisoner is released from custody,”
§3624(b)(2), meaning that it can be revoked at any time
before that date. This gives prisoners approaching their
release date an extra incentive to behave.
   As a fallback, the Court wonders what would happen if
a prisoner misbehaved on the final day of his ten-year
sentence. Would the Bureau of Prisons (BOP) be forced to
“retroactively adjust the duration of all of his [term years]
to 365 days”? Ante, at 17. The answer is what one might
suppose: A prisoner whose credits are revoked will find
himself precisely where he would have been if those cred
its had never been earned. All years of the term remain
365 days, as they always have. But a misbehaving pris
oner who had formerly earned, say, 500 credits will find
himself without the benefit of those 500 days. That will
leave him with more of his term to complete—500 days
more, to be precise. If he behaves well again, he can re
sume earning credit for the remainder of his term, but he
has lost the opportunity to earn credits for any prior years.
See §3624(b)(1). This is not at all confusing for a prisoner;
and certainly it is as straightforward, if not more so, than
the Court’s approach. The Court’s view causes a prisoner’s
“term of imprisonment” to shrink over time according to
an algebraic formula, only to expand again if he misbe
8                    BARBER v. THOMAS

                    KENNEDY, J., dissenting

haves.
   Finally, the Court speculates that BOP might find the
above approach difficult to administer. The Court identi
fies no basis for this claim, nor does one exist. The infor
mation used to calculate a prisoner’s term under the above
approach is the same as it is under the Court’s approach.
True, a prisoner may become eligible to be awarded credit
on different calendar days during the course of his term.
But under the Court’s approach, this also happens when
awarding credit in the final year. And, it goes without
saying, federal prisoners begin their incarceration on
different calendar days anyway, so that under any ap
proach, BOP will be forced to evaluate prisoners through
out the calendar year.
                               II
   The Court’s reading of §3624(b)(1), therefore, is less
consistent with the text than the reading explained above.
But even if these interpretations were in equipoise, under
any fair application the rule of lenity should tip the bal
ance in petitioners’ favor. When a penal statute is suscep
tible of two interpretations, the one more favorable to the
defendant must be chosen unless “text, structure, and
history . . . establish that the [harsher] position is unam
biguously correct.” United States v. Granderson, 511 U. S.
39, 54 (1994). Resolving ambiguity in favor of lenity en
sures that statutes provide “fair warning[,] . . . in language
that the common world will understand, of what the law
intends to do if a certain line is passed.” United States v.
Bass, 404 U. S. 336, 348 (1971) (internal quotation marks
omitted). The rule thus applies “not only to interpreta
tions of the substantive ambit of criminal prohibitions, but
also to the penalties they impose.” Bifulco v. United
States, 447 U. S. 381, 387 (1980).
   The Court assumes without deciding that §3624(b) is
penal in nature. See ante, at 13. No assumption is neces
                 Cite as: 560 U. S. ____ (2010)           9

                    KENNEDY, J., dissenting

sary: The statutory provision awarding good time credits
“in fact is one determinant of [a] prison term,” so that a
prisoner’s “effective sentence is altered once this determi
nant is changed.” Weaver v. Graham, 450 U. S. 24, 32
(1981). In Weaver, the Court considered whether an
amendment to Florida’s statutory formula for calculating
good time credits implicated the Ex Post Facto Clause.
The Court concluded that it did, as the new statute “sub
stantially alter[ed] the consequences attached to a crime
already completed, and therefore change[d] ‘the quantum
of punishment.’ ” Id., at 33 (quoting Dobbert v. Florida,
432 U. S. 282, 294 (1977)). For the same reason, the penal
effect of §3624(b)(1) is substantial enough to implicate the
rule of lenity. We should not disadvantage almost 200,000
federal prisoners unless Congress clearly warned them
they would face that harsh result.
                              III
   The Government—although not the Court—argues that
we should embrace its interpretation out of deference to
BOP. BOP has been charged by the Attorney General
with responsibility for “[a]pproving inmate disciplinary
and good time regulations.” 28 CFR §0.96(s) (2009). BOP
has long followed the same credit-calculation method now
advocated by the Court. The Government argues that we
should defer to BOP’s choice as a permissible exercise of
its delegated responsibility.
   This argument fails on multiple levels. There is no
indication that BOP has exercised the sort of interpretive
authority that would merit deference under Chevron
U. S. A. Inc. v. Natural Resources Defense Council, Inc.,
467 U. S. 837 (1984). The statute does not create a legis
lative gap for BOP to fill. To the contrary, the procedures
that govern the timing of credit awards are spelled out in
great detail. Cf. Lopez v. Davis, 531 U. S. 230, 241–242
(2001) (where statute says that BOP “may” grant early
10                  BARBER v. THOMAS

                    KENNEDY, J., dissenting

release to certain prisoners, without specifying further
criteria, Congress deliberately created a “statutory gap”).
The statute even goes so far as to explain what to do “[i]f
the date for a prisoner’s release falls on a Saturday, a
Sunday, or a legal holiday.” §3624(a). This legislative
specificity as to timing contrasts with other provisions
that do delegate authority to BOP. E.g., §3624(b)(1)
(awarding of credit is “subject to determination” by BOP
that the prisoner “has displayed exemplary compliance
with institutional disciplinary regulations”).
   BOP has not claimed that its view is the product of any
“formal administrative procedure tending to foster the
fairness and deliberation that should underlie a pro
nouncement” with the force of law. United States v. Mead
Corp., 533 U. S. 218, 230 (2001). In 2005, BOP made final
an administrative rule adopting its preferred methodology.
70 Fed. Reg. 66752 (adopting 28 CFR §523.20). But when
pressed during an earlier stage of this litigation, BOP
conceded that it had “failed to articulate in the adminis
trative record the rationale upon which it relied when it
promulgated” the rule. Tablada v. Thomas, 533 F. 3d 800,
805 (CA9 2008). The Court of Appeals accepted BOP’s
concession, ibid., and that aspect of its ruling has not been
appealed.
   As a fallback position, the Government argues that
BOP’s interpretation should receive at least some defer
ence under Skidmore v. Swift & Co., 323 U. S. 134 (1944).
But under Skidmore, an agency decision only merits “re
spect proportional to its ‘power to persuade.’ ” Mead,
supra, at 235 (quoting Skidmore, supra, at 140). BOP’s
position is of long standing, but the administrative record
is noteworthy for what it does not contain—namely, any
reasoned justification for preferring BOP’s methodology
over statutorily permissible alternatives. BOP has consis
tently adhered to its mistaken belief that its approach is
the only one that can be squared with the text. See 62
                 Cite as: 560 U. S. ____ (2010)          11

                    KENNEDY, J., dissenting

Fed. Reg. 50786 (1997) (explanation to interim rule assert
ing that the correct methodology “had been clearly stated
by statute since the implementation of the Sentencing
Reform Act of 1984”). For example, at no point did BOP
consider, much less consciously reject, the interpretation
outlined here. Cf. Reno v. Koray, 515 U. S. 50, 60–61
(1995) (deferring to BOP’s reasoned decision to reject one
interpretation in favor of another). An agency need not
consider all possible alternatives. But deference is not
owed to an agency view, however consistently held, that
from the start has been premised on legal error. See
Mead, supra, at 228; Skidmore, supra, at 140.
                        *     *     *
  The straightforward interpretation urged here accords
with the purpose of the statute, which is to give prisoners
incentive for good behavior and dignity from its promised
reward. Prisoners can add 54 days to each year. And
when they do so, they have something tangible. In place
of that simple calculation, of clear meaning, of a calendar
that can be marked, the Court insists on something differ
ent. It advocates an interpretation that uses different
definitions for the same phrase in the same sentence;
denies prisoners the benefit of the rule of lenity; and caps
off its decision with an appendix that contains an alge
braic formula to hang on a cell wall.
  To a prisoner, time behind bars is not some theoreti
cal or mathematical concept. It is something real, even
terrifying. Survival itself may be at stake. See Dept.
of Justice, Bureau of Justice Statistics, C. Mumola,
Suicide and Homicide in State Prisons and Local
Jails (NCJ 210036, Aug. 2005), online at http://
bjs.ojp.usdoj.gov/content/pub/pdf/shsplj.pdf (all Internet
materials as visited June 2, 2010, and available in
Clerk of Court’s case file) (prison homicide rates); Na
tional Prison Rape Elimination Commission Re
12                  BARBER v. THOMAS

                   KENNEDY, J., dissenting

port, p. 4 (June 2009) (citing a national survey estimating
that 60,500 state and federal prisoners had been sexually
abused during the preceding year). To this time, the
Court adds days—compounded to years. We should not
embrace this harsh result where Congress itself has not
done so in clear terms. I would reverse the judgment of
the Court of Appeals.
