How Much Time Do You Serve on an 18-Month Federal Sentence?

On an 18-month federal sentence, you will most likely serve about 15 months and 9 days in custody, and possibly less time inside an actual prison if you qualify for First Step Act credits that move you to a halfway house or home confinement for the final stretch. How much time you actually serve on an 18-month federal sentence comes down to two credit systems and one placement decision, all controlled by the Bureau of Prisons. The federal system abolished parole in 1987, so the sentence the judge announced is the sentence you serve, reduced only by specific statutory credits.¹

Good Conduct Time Takes About 81 Days Off

The biggest reduction comes from Good Conduct Time. Federal law allows inmates serving more than a year to earn up to 54 days of credit for each year of the sentence the judge imposed.² The BOP prorates the credit for partial years.³

For 18 months, multiply 54 by 1.5. That is 81 days, or roughly 2 months and 21 days. Subtract that from 18 months and your projected release lands at about 15 months and 9 days. That works out to about 85 percent of the imposed sentence, which is why people describe the federal system as an 85-percent system.

Full credit is not automatic. The statute requires “exemplary compliance with institutional disciplinary regulations,” and the BOP also considers whether you are making progress toward a GED or high school diploma if you don’t already hold one.² Most inmates who stay out of trouble receive the full 54 days per year. An incident report for fighting, contraband, or a failed drug test can cost you some or all of the credit for that period, and once forfeited it cannot be restored.

First Step Act Credits Can Shorten Time Inside a Facility

On top of Good Conduct Time, the First Step Act created a separate credit called Earned Time Credits. These do not shorten the sentence itself. They apply toward earlier transfer into prerelease custody, meaning a halfway house or home confinement, or in some cases earlier release to supervised release.⁴

The rate depends on your risk classification. Standard participants earn 10 days of credit for every 30 days of successful programming. Inmates the BOP classifies as minimum or low risk of recidivism, and who hold that classification across two consecutive assessments, earn 15 days per 30-day period.⁴

Qualifying activities are drawn from the BOP’s approved Evidence-Based Recidivism Reduction programs and Productive Activities. Examples include the Residential Drug Abuse Treatment Program, vocational and occupational education, the Bureau Literacy Program for those pursuing a GED, anger management, and the UNICOR work program.⁵ The BOP assigns programming based on your individual needs assessment.

Not everyone can earn these credits. The First Step Act disqualifies inmates convicted of dozens of offenses, including terrorism and weapons crimes, sexual exploitation and trafficking, espionage, certain violent offenses, and some immigration offenses.⁶ If your offense is on that list, you can still participate in programming but you will not earn Earned Time Credits for it. Good Conduct Time remains available regardless of offense type.

Halfway House and Home Confinement at the End

Federal law directs the BOP to move inmates into the community during the final portion of a sentence, up to a maximum of 12 months before release, and that time can be spent at a Residential Reentry Center (a halfway house) or on home confinement.²

A Residential Reentry Center offers structured supervision. You live at the facility but can leave for work, medical appointments, and approved activities. Home confinement keeps you at your residence with location monitoring. Under the general prerelease authority in the statute, home confinement is limited to the shorter of 10 percent of your sentence or six months.² For 18 months, 10 percent is about 54 days. First Step Act credits can extend home confinement placement beyond that baseline.

A May 2025 BOP directive made home confinement the default pathway for eligible inmates who don’t need the structured support of a halfway house, and confirmed that there is no cap on the number of First Step Act credits that can be applied toward home confinement.⁷ A low-risk inmate on an 18-month sentence who has been steadily earning credits can spend a meaningful portion of the back end at home rather than behind the walls.

Pretrial Detention Counts

Time you spent in federal custody before sentencing counts against your sentence. Federal law requires the BOP to credit pretrial detention as long as the detention was connected to the offense you were sentenced for and the time hasn’t already been applied to a different sentence.⁸

This changes the math meaningfully. Someone who sat in a detention center for three months before receiving an 18-month sentence has already served three months when they arrive at their designated facility. If you think the BOP miscalculated your credit, raise it with the facility’s Correctional Systems staff, and if that doesn’t resolve it, use the Administrative Remedy Program with help from your unit team.⁹

Realistic Timeline for 18 Months

For someone with no pretrial detention credit and clean conduct throughout:

Sentence imposed: 18 months, roughly 547 days.
Maximum Good Conduct Time: 81 days.
Projected release with full Good Conduct Time: about 15 months and 9 days.
First Step Act credits, if eligible, apply toward earlier prerelease placement or supervised release.
The final weeks or months may be served at a halfway house or on home confinement instead of in a prison.

The floor for total federal custody, in some form, is about 15.3 months for an inmate who earns all available Good Conduct Time. Time actually inside a prison can be shorter than that if First Step Act credits move you to home confinement sooner. A low-risk inmate who pursues programming aggressively from the start and earns credits at the 15-day rate can transition out of a facility well before the 15-month mark.

The worst case runs the other direction. An inmate who racks up disciplinary infractions and loses Good Conduct Time can serve close to the full 18 months. The gap between best-case and worst-case is wide enough that day-to-day conduct inside is the single most consequential thing you control.

The Sentence Doesn’t End at the Prison Gate

Federal sentences almost always include a term of supervised release that begins the day you leave custody. It runs alongside your reentry, not against your prison time, but it is worth knowing about before you plan around a release date. The maximum length depends on the underlying offense class: up to 5 years for a Class A or B felony, up to 3 years for a Class C or D felony, and up to 1 year for a Class E felony or misdemeanor.¹⁰ Violating a condition can send you back to prison on top of any new charges.¹¹

1
Justia. Criminal Law Authority – Sentencing Guidelines
2
Office of the Law Revision Counsel. 18 USC 3624 – Release of a Prisoner
3
eCFR. 28 CFR 523.20 – Good Conduct Time
4
Office of the Law Revision Counsel. 18 USC 3632 – Development of Risk and Needs Assessment System
5
Federal Bureau of Prisons. Evidence-Based Recidivism Reduction Programs and Productive Activities
6
Federal Bureau of Prisons. Time Credits Disqualifying Offenses
7
Federal Bureau of Prisons. Federal Bureau of Prisons Issues Directive to Expand Home Confinement, Advance First Step Act
8
Office of the Law Revision Counsel. 18 USC 3585 – Calculation of a Term of Imprisonment
9
Federal Bureau of Prisons. Sentence Computations
10
Office of the Law Revision Counsel. 18 USC 3583 – Inclusion of a Term of Supervised Release After Imprisonment
11
United States Sentencing Commission. Guidelines Manual Chapter 7 – Violations of Probation and Supervised Release