In this paper, an integral methodology for the modeling of a twin-screw compressor is presented. Starting from a known rotor profile, all the algorithms to calculate the second rotor profile, the size of the control volume, and the compressor's performance are presented. The proposed modeling approach can be applied in an optimization procedure to find the optimal rotor profiles for a given application, with corresponding working conditions. Furthermore, based on the modeling results and substantiated with measurements on different compressor types, a similarity law for positive displacement compressors seems to exist. The existence of a similarity law has large application potential as it could be used to predict the performance of a positive displacement compressor in other than the (lab) tested working conditions. Further investigation of the similarity law for positive displacement compressors is therefore proposed as a key topic for future research.