The size of a prisoner population is a function of the rate of entry to it and the length of time prisoners stay. This analysis assumed a stable institutional population; the average length of stay was derived from the prison population divided by the number of prisoner commitments per unit of time. Sentenced prisoner populations in New South Wales and Victoria differ partially due to different sentencing policies; New South Wales uses periodic detention as a sentencing option, while Victoria does not, and sentences a greater number of fine defaulters. When all sentenced prisoners are examined, the New South Wales prisoner population rate is about 2.5 times than in Victoria, but the average length of stay is about 2.5 months shorter. When periodic detainees are excluded from the analysis, the New South Wales prisoner rate drops to double that of Victoria. Excluding imprisoned fine defaulters does not alter the figures significantly. 3 tables and 17 notes