There are four variables involved in calculating a federal prison sentence.
1.Good Time Credit-
This is the time that the BOP gives back to inmates who follow the rules and stay out of trouble. Under the Sentencing Guidelines it is equal to 15% of time served. Anyone sent to 12 months or less will not qualify for any Good Time Credit. This is why sentences of 12 months and 1 day are often handed down. It is the judge's way of showing some leniency.
Rather than just subtracting 15% from the sentence, the BOP has chosen to employ some interesting logic in interpreting the meaning of 15%. This is explored in further detail below. This common misunderstanding means that subtracting 15% of the total sentence duration will not result in an accurate calculation of Good Time Credit to be awarded.
2.Residential Drug Abuse Program (RDAP) –
Those inmates who are placed in and successfully complete the BOP's Residential Drug Abuse Program can qualify for a sentence reduction of up to one year. Placement is reserved for those who are deemed to have severe drug abuse problems.
The BOP's uses halfway houses to integrate offenders back into the community, before they are released. While no longer imposed, offenders in a halfway house are subject to a plethora of rules and regulations and are still under the jurisdiction of the BOP.
An inmate is eligible for a halfway house with 10% remaining on their sentence or 6 months in the case that 10% is greater than 6 months. Halfway house time is based on actual time served, not on the total duration of the sentence. Once an inmate becomes eligible for a halfway house, he or she is in no way guaranteed immediate placement. However, especially on sentences of over 12 months, some halfway house time can be expected. Halfway house time is extremely rare on sentences of less than 6 months.
4.Time Already Served-
Time already served in custody awaiting sending or during transport to the designated prison, must be deducted. This time is subtracted after the above calculations have been made eg if shipment is 366 days and actual time to be served is 319 days and time already served is 3 days, the 3 days would be deducted from 319 not from 366.
The task of Calculating Good Time Credit to be awarded is probably the most confusing and difficult of the above four steps. This stems from the fact that while the Sentencing Guidelines called for a well behaved inmate to serve 85% of his sentence, the BOP chose to interpret this differently. Instead of simply subtracting 15% of the total sentence, the BOP decided to base the reduction on the time that would have been served under the system prior to the Sentencing Guidelines and Good Time Credit.
In 1987 the Sentencing Commission's staff was tasked with the job of creating a baseline for the Sentencing Table upon which all federal provisions would be graphed. They collected a large sample of sentences for a broad array of crimes and determined the actual time served as a baseline. When the sentence was more than 12 months, the commission divided by .85. In doing so, the sentences in the Levels Table, which was used prior to Good Time Credit, became comparable with those in the new Guidelines.
In essence then, the BOP succeeded in adhering to the new Guidelines, while at the same time keeping inmates incarcerated beyond the time that many advocates insist was the intent of those very Guidelines.