A mathematical model is presented in this work, based on the Blade Element Momentum (BEM) theory for the horizontal-axis wind turbine design, taking into account the influence of the wake on the rotor plane in the general form. This influence is considered when the tip-speed-ratio is small, justifying the development of formulations that predict the effects of the wake on the rotor plane. The proposed mathematical model in this work is an extension of the BEM method, using the Glauert's model modified for the wind turbine design. (C) 2010 Elsevier Ltd. All rights reserved.