How Much of Your Sentence Do You Serve in Illinois?

How much of your sentence you serve in Illinois depends almost entirely on the offense you were convicted of. Most felonies are served at 50% with day-for-day good-conduct credit. Certain large-scale drug crimes require 75%. A list of violent offenses under truth-in-sentencing requires 85%. First-degree murder and terrorism require 100%. Pretrial jail time and program participation can shave off more, but only within the ceiling your offense category sets.

Time You Already Served Comes Off First

Before any good-conduct math runs, Illinois subtracts the days you spent locked up waiting for your case to resolve. Five months in county jail before sentencing on a three-year prison term means those five months come off the top.¹ The credit applies at the same rate used for your offense category, so a pretrial day on a 50% offense earns the same day-for-day credit as a day in prison.

Home confinement counts too. Days spent on a curfew of twelve or more hours, or on electronic monitoring that restricted your movement, qualify as custodial time. Courts can also credit days in pretrial psychiatric or substance abuse treatment if the confinement was custodial in nature.¹

The 50% Default for Most Felonies

For most Illinois felonies, IDOC applies what it calls Statutory Sentence Credit. You earn one day of credit for each day served without major disciplinary trouble, which cuts prison time roughly in half.² A six-year burglary sentence works out to about three years actually served, assuming good behavior.

You don’t apply for this credit. IDOC calculates it automatically. It can also be taken away. Serious rule violations in prison can strip accumulated credit and push your release date back. The Department prescribes the rules for awarding and revoking credit, subject to review by the Prisoner Review Board.³

75% Offenses: Large-Scale Drug Crimes

A step up from the default, certain drug offenses cap good-conduct credit at 7.5 days per month, so you serve at least 75% of the sentence. The list targets large-scale operations: gunrunning, narcotics racketeering, drug-induced homicide, controlled substance trafficking, and methamphetamine trafficking. Some Class X drug convictions also fall here when the substance weighs 100 grams or more.³

People in this tier can bring their time served down to 60% by earning program credit. Gunrunning is the one exception, with a hard floor at 75% regardless of programming.³

85% Offenses Under Truth-in-Sentencing

Illinois truth-in-sentencing rules require at least 85% of the term to be served on a specific list of violent offenses. Good-conduct credit is capped at 4.5 days per month.

Some offenses fall into the 85% tier automatically: attempted first-degree murder, solicitation of murder, predatory criminal sexual assault of a child, aggravated criminal sexual assault, criminal sexual assault, aggravated kidnapping, aggravated battery with a firearm, heinous battery, aggravated battery of a senior citizen, aggravated battery of a child, aggravated domestic battery, aggravated discharge of a firearm, unlawful possession of a firearm by a repeat felony offender, and a second or subsequent offense of luring a minor.³

Others land at 85% only when the sentencing court specifically finds the offense caused great bodily harm to a victim: home invasion, armed robbery, aggravated vehicular hijacking, and armed violence with a category I or II weapon.³ That finding matters. An armed robbery with no serious injury drops back to 50%. The same conviction with a great-bodily-harm finding locks in at 85%. Defense attorneys fight over that finding at sentencing for exactly this reason.

100% Offenses

For the most serious convictions, Illinois eliminates sentence credit entirely. Someone convicted of first-degree murder or terrorism serves every day the judge imposes. A natural life sentence means exactly that.³ The sentencing judge must state on the record that no sentence credit of any kind will be available.

Program Credit on Top of the Default

Beyond automatic day-for-day credit, IDOC offers Earned Program Sentence Credit for rehabilitation programming.² The rate depends on the program.

Full day-for-day credit is available for substance abuse treatment, educational programs, vocational training, correctional industry assignments, behavior modification programs, life skills courses, and reentry planning. Up to half a day per day is available for self-improvement programs, volunteer work, and work assignments that don’t qualify at the higher rate.³

These credits are earned, not automatic. You have to enroll, participate, and satisfactorily complete the program by IDOC standards. Programming completed in county jail before sentencing also counts.

How Program Credit Interacts With the Tiers

Since January 1, 2018, people convicted of 75%, 85%, and 100% offenses can technically earn program credit, but it cannot push their time served below certain floors.³

On 75% offenses, program credit can bring you to 60%. Gunrunning stays at 75%.
On 85% offenses, the floor is 85%, so program credit provides no practical additional reduction.
On 100% offenses, the floor is 100%. No program credit changes the outcome, and anyone serving natural life cannot receive program credit at all.

In practice, program credit makes the biggest difference for 50% sentences and for people serving 75% terms.

A Separate Path for Offenses Committed Before Age 21

People who committed their offenses before turning 21 have a separate route to potential release, independent of the credit tiers above. Under 730 ILCS 5/5-4.5-115, someone sentenced on or after June 1, 2019 can petition the Prisoner Review Board for a parole hearing. When you become eligible depends on the offense:⁴

After 10 years for most offenses, but not first-degree murder, aggravated criminal sexual assault, or predatory criminal sexual assault of a child.
After 20 years for first-degree murder and aggravated criminal sexual assault, but not predatory criminal sexual assault of a child or natural life sentences.
After 40 years even for natural life sentences, with predatory criminal sexual assault of a child again excluded.

Predatory criminal sexual assault of a child disqualifies a youthful offender from parole review at every tier. Eligibility for a hearing is not a guarantee of release. The Board weighs rehabilitation, institutional conduct, and reentry plans before deciding.

The Sentence Doesn’t End at the Prison Gate

Walking out of an Illinois prison does not finish your sentence. Nearly every felony carries a term of Mandatory Supervised Release, which works like parole. You serve it in the community under IDOC supervision, and it is part of the original sentence, not an add-on.

Length depends on felony class:

Sex offenses run on a different scale. Predatory criminal sexual assault of a child, aggravated criminal sexual assault, or criminal sexual assault carry MSR terms ranging from a minimum of 3 years up to the natural life of the defendant.⁶ A second or subsequent conviction for aggravated criminal sexual abuse or felony criminal sexual abuse involving a minor carries a 4-year MSR term, with at least the first 2 years on electronic monitoring or home detention.

Violating MSR Can Send You Back

An MSR violation goes before the Prisoner Review Board. You are entitled to a preliminary hearing unless the violation involves new criminal charges where probable cause has been found, or a new conviction.⁷

If the Board revokes your MSR, you return to prison for the remainder of the MSR term, minus any time between release and the violation. The Board can also order you to serve up to one additional year of the original court-imposed sentence that you avoided through accumulated sentence credit.⁷ You get credit for any custody time since release that hasn’t been applied elsewhere. A lot of people end up back inside through MSR revocation, and the reconfinement math catches many off guard because they assumed the sentence was finished.

1
Illinois General Assembly. Illinois Code 730 ILCS 5/5-4.5-100 – Credit; Time in Custody
2
Illinois Department of Corrections. Sentence Calculation for Individuals in Custody
3
Illinois General Assembly. Illinois Code 730 ILCS 5/3-6-3 – Rules and Regulations for Sentence Credit
4
Illinois Prisoner Review Board. Youthful Parole Guidelines
5
Illinois General Assembly. Illinois Code 730 ILCS 5/5-4.5-25 – Class X Felonies; Sentence
6
Illinois General Assembly. Illinois Code 730 ILCS 5/5-8-1 – Natural Life Imprisonment; Enhancements for Use of a Firearm; Mandatory Supervised Release Terms
7
Illinois General Assembly. Illinois Code 730 ILCS 5/3-3-9 – Violation; Revocation of Parole or Mandatory Supervised Release; Hearing