Gerotor pumps are positive displacement pumps used in many applications thanks to their compact size and low cost. In order to study new designs and their performances, a detailed and robust model can be used. In our previous paper ( Totaro, G., Zardin, B., Borghi, M., Scolari, F.: Modelling of a Gerotor pump including the evaluation of the micro-movements of the external gear. J. Phys.: Conf. Ser. 2648, 012,049 (2023). https://doi.org/10.1088/1742-6596/2648/1/012049 .), we had developed a lumped parameter model to evaluate the Gerotor pump’s performance. This work continues the development of the model, by adding and discussing a simple method to estimate the viscous losses of the machine. This method should be used in combination with our previous lumped parameter model to obtain an estimation of the overall pump performance or independently to evaluate only the mechanical performance. Along the paper, each contribution of loss is presented and discussed, and its contribution to the total losses for different operating conditions is explored. We have also analyzed the influence of the lubricated clearances heights on the losses. The torque losses analyzed in the model are the viscous losses due the lubricated interface between the outer gear and the housing, the losses of the journals bearings of the shaft and the viscous losses due the lateral lubricated interface between the gears faces and the housing. To evaluate the torque, few information are required: geometry and tolerances of the components of the pump, fluid properties and operating conditions. In conclusion, we propose a simple torque evaluation method that can be also useful to individuate the main losses that influencing the hydro-mechanical efficiency of the pump.