In this article, the methodology to project prison population starting from a projected number is presented. Several items are identified as prerequisites to produce a prison population projection model. First, a good understanding of the penal code section that regulates time served for offenders is necessary. Second, information about the most recent admissions is needed. Third, information on releases from prison is needed. Using this information, the survival tables are produced. Finally, a point must be made regarding legislative changes and prison admissions. When changes are made that affect the amount of time served in prison, the admissions module must incorporate those changes. The projected total prison population is the result of (a) the number produced from projected population from admissions (Module 2) and (b) the number of offenders remaining from the population who were in prison at the time the projection began (Module 3). During periods when States are suffering budget shortfalls, it is important to have some reliable manner of conducting projections firmly based on existing policies. A dynamic model allows changing parameters to answer “what if” types of questions. In addition, an intuitive model refers to the ability of the model to be understood by individuals who are not “number oriented.” However, there needs to be a tool that can be used as a didactic aid to show how the criminal justice system in a particular State works and how changes in a part of the system affect the entire system. The model is a disaggregated flow simulation model that uses Microsoft Excel. Offenders are grouped into unique categories that share common release laws, based on offense categories and sentence lengths. Figure, tables and references