If a person is senteced to 18 months in a federal prison how long will the person have to serve?

In federal prison, inmates typically serve about 85% of their sentence due to good time credits, unless they have a disciplinary infraction. For an 18-month sentence, this means the person would likely serve approximately 15.3 months. However, factors such as behavior and program participation can influence the exact duration. After serving their time, they may also be subject to supervised release.