"At yearend 2015, the United States had an estimated 1,526,800 prisoners under the jurisdiction of state and federal correctional authorities. This was the smallest U.S. prison population since 2005 (1,525,900 prisoners). The prison population decreased by more than 2% from the number of prisoners held in December 2014 (figure 1). This was the largest decline in the number of persons under the jurisdiction of state or federal correctional authorities since 1978 (figure 2). [...] The statistics in this report are based on the Bureau of Justice Statistics' (BJS) National Prisoner Statistics (NPS) Program, which collects annual data from state departments of corrections (DoCs) and the BOP (Bureau of Prisons) on prisoner counts, prisoner characteristics, admissions, releases, and prison capacity."
Bureau of Justice Statistics: http://www.bjs.gov/