The basic component of the methodology is a projection of future prison populations under existing sentencing practices and under sentencing practice revisions. The number of prison commitments is projected by multiplying the size of each jurisdictional subgroup by crime arrest and incarceration rates. Average time served in prison is also determined for the jurisdiction. The methodology was applied to impacts on the Pennsylvania State prison system to compare the effects of a mandatory-minimum sentencing bill with practices currently in use. Increases in State prison populations were projected at over 50 percent if mandatory sentencing were implemented. Implementation of minimum only sentencing processes would result in an 8 percent increase in the prison population. Judges would, therefore, be more likely to use the minimum sentencing provisions of the bill than the mandatory ones. The methodology assumes that full legal compliance will occur and that criminal justice practices will remain unchanged. Tables, notes, and 24 references are included.