How much time will they actually serve?

Good conduct time. For a sentence of more than 1 year, the Bureau of Prisons can take up to 54 days off for each year the judge imposed.¹1 18 U.S.C. § 3624(b)(1) (sentence of more than 1 year, other than life; up to 54 days for each year of the sentence imposed). On day one it counts the most good time the sentence can earn and sets a projected release date from that. A part-year at the end earns its share, and part-days are dropped.²2 28 C.F.R. § 523.20(b)(1)–(2); Good Conduct Time Credit Under the First Step Act, 87 Fed. Reg. 7938 (Feb. 11, 2022) (a 24-month-and-1-day sentence earns 108 days: partial days are rounded down). That is where the “85%” people talk about comes from. Someone without a high school diploma or GED who isn’t working toward one earns up to 42 days a year instead of 54, unless they are a non-citizen under a final order of removal, who keeps the 54-day rate.³3 28 C.F.R. § 523.20(d)(2)–(3); 18 U.S.C. § 3624(b)(1). Discipline can take good time away.

Jail time before sentencing. Days locked up on the case before the sentence began count toward it, as long as they weren’t counted toward another sentence.⁴4 18 U.S.C. § 3585(b).

First Step Act time credits. Someone whose conviction isn’t on the excluded list earns 10 days of credit for every 30 days of programs or productive activities, and 15 days once two risk assessments in a row come back minimum or low.⁵5 18 U.S.C. § 3632(d)(4)(A); 28 C.F.R. § 523.42(c). Under the BOP rule that took effect on September 30, 2026, credits start once the term of imprisonment begins, not when they reach the prison where they will serve it. For someone already in custody that is usually the day they were sentenced.⁶6 First Step Act Time Credits—Revisions, 91 Fed. Reg. 55740 (Aug. 31, 2026) (interim final rule, effective Sept. 30, 2026); 28 C.F.R. § 523.42(a); 18 U.S.C. §§ 3585(a), 3632(d)(4)(B).

How credits are used. Credits can only be applied for someone at minimum or low risk, or with a petition the warden approves.⁷7 18 U.S.C. § 3624(g)(1); 28 C.F.R. § 523.44(b)–(c). The first 365 days go to an earlier start of supervised release, if the judgment includes supervised release. Credits past that move them to a halfway house or home confinement once the credits equal the time left.⁸8 18 U.S.C. § 3624(g)(3), (g)(10); 28 C.F.R. § 523.44(d); 91 Fed. Reg. 55740 (the first 365 days to early supervised release; credits beyond that to prerelease custody). Someone under a final order of removal can earn credits but can’t use them.⁹9 18 U.S.C. § 3632(d)(4)(E); 28 C.F.R. § 523.44(a)(2).

The excluded list. The law lists about 68 kinds of convictions that can’t earn credits at all.¹⁰10 18 U.S.C. § 3632(d)(4)(D)(i)–(lxviii); see (xxii) (§ 924(c)), (lxv)–(lxvii) (heroin, fentanyl, methamphetamine). Three of them are often misunderstood: a conviction under 18 U.S.C. § 924(c) (a gun carried or used during a drug or violent crime) is on the list, fentanyl trafficking under 21 U.S.C. § 841(b)(1)(A)(vi) or (B)(vi) is on it, and heroin and meth trafficking are on it only when the judge found the person was an organizer, leader, manager or supervisor. This calculator doesn’t decide that from the charge. It asks, and shows both answers if you don’t know.

The Second Chance Act. Separately, the BOP may use up to the last 12 months for a halfway house, and home confinement for the shorter of 10% of the sentence or 6 months. It decides how much, and it can be none.¹¹11 18 U.S.C. § 3624(c)(1)–(2).

• It is not the BOP’s computation. The official dates are on the sentence computation the BOP prepares. Ask the unit team (the case manager) for a copy, and check it against this.
• It assumes the best case. Full good time, programs every month, and no waiting lists. Lost good time, a gap in programs, or a time in a status that doesn’t earn credits all push the date later.
• It doesn’t guess the risk level. The BOP scores it with its own tool, and it can change at each reassessment.
• It leaves out the drug program. Finishing the BOP’s residential drug program can take up to 1 more year off for someone convicted of a nonviolent offense,¹²12 18 U.S.C. § 3621(e)(2)(B). so ask about it early.
• It handles one federal sentence. Consecutive sentences, state sentences, detainers and resentencing all change the math.
• The rules are moving. The September 2026 rule is an interim rule, and courts are still deciding some First Step Act questions. This page was checked on October 30, 2026.