he way that the Federal Bureau of Prisons calculates the reduction for good behavior, it comes out to more than 85% of the sentence. It is really about 87%. The problem is that the prisoner gets a time reduction only after serving his first year of imprisonment.

If a person has a GED or high school diploma, the maximum credit they earn per year is about 47 days off their sentence. If they do not have a GED or high school diploma, a prisoner will earn a maximum of about 42 days off their sentence per year.

There is a good explanation here -although I will tell you the way the Federal Bureau of Prisons calculates good time days earned is confusing and not exactly logical.

A 12 year plus 1 day sentence is 4,381 days. If they earn 47 days a year after that first year, they will get 527 days off their sentence, or approximiately 1 year and 5 months off their sentence.