This paper develops a new approximate model to predict the pressure and momentum forces on a Savonius-style vertical axis wind turbine. Flow distributions through and around the turbine are examined for analytical predictions of the torque and power output, at all rotor angles. A new approximate streamtube method is developed to predict the momentum, lift, and drag forces on the rotor surfaces by the air stream on the basis of an integral force balance on the turbine blades. Unlike other past analytical methods, the technique predicts both momentum and pressure forces imposed on the rotor surface during operation. The calculated results are validated against experimental data and numerical predictions from computational fluid dynamics. Copyright (c) 2012 John Wiley & Sons, Ltd.