This paper describes and discusses a numerical approach to predict windage losses inside high-speed permanent magnets synchronous machines. The air flow is thoroughly analyzed into a rotor-stator system which consists of two concentric cylinders where the inner cylinder is rotating and the outer cylinder is stationary. For the different studied cases, only smooth air gaps are considered and isothermal boundary conditions are imposed on both rotor and stator surfaces. Based on CFD modeling, rotor skin friction coefficients are estimated numerically for different air gap geometries which are used then to estimate windage losses in a large operation range (high Reynolds numbers). The numerical results obtained in this paper are used to define a piecewise correlation to fit well the rotor skin friction coefficients for different air gap geometries and different operation conditions. A comparison between numerical and experimental data is proposed and a reasonable agreement is found.