This paper will present an analysis of the effects of scale on the separation efficiency of inertial separators with emphasis on aircraft engine inlet particle separator (IPS) applications. A similarity analysis is described which takes into account the effect of IPS size on the separation process within the IPS and the effect of size on the ratio of particle accelerations to drag-to-weight. A similarity curve is defined which can then be used to extrapolate separation efficiency to geometrically similar IPS's of different sizes, air flow and various particle distributions. The analysis is applied to three IPS's designed for engines that span the full spectrum of aircraft and auxiliary power units turboshaft applications: the Pratt and Whitney PW3005, the AVCO/PW T800-APW-800 and the Turbomach MPSPU T100. The analysis correctly predicts the increases in separation efficiencies with AC Coarse test dust with smaller IPS's and that separation efficiency with the larger military specification sand is relatively insensitive to IPS size.