A computer model for calculating screw compressor performance is presented. Geometrical characteristics such as volume curve, sealing line length, discharge port area, etc. are studied. The volume curve is obtained from the sealing line shape, using the principle of virtual work. This procedure has the advantage of simplifying the numerical calculation of the volume curve. An analytical model of an oil-injected screw compressor is developed, based on the laws of thermodynamics for perfect gases. The effects of internal leakage 1 heat exchange between gas and oil, and flow resistance at suction and discharge ports are included in the model. Some numerical examples of the P-V diagram, volumetric efficiency and adiabatic efficiency for sample rotors are demonstrated for various leakage areas and rotor wrap angles.