A new approach based on the open source tool OpenFOAM is presented for the numerical simulation of a mini gerotor pump working at low pressure. The work is principally focused on the estimation of leakage flow in the clearance disk between pump case and gears. Two main contributions are presented for the performance of the numerical simulation. On one hand, a contact point viscosity model is used for the simulation of solid–solid contact between gears in order to avoid the teeth tip leakage. On the other hand, a new boundary condition has been implemented for the gear mesh points motion in order to keep the mesh quality while moving gears with relative velocity. Arbitrary coupled mesh interface (ACMI) has been used both in the interface between clearance disk in inlet/outlet ports and between clearance disk and interteeth fluid domain. Although the main goal of the work is the development of the numerical method rather than the study of the physical analysis of the pump, results have been compared with experimental measurement and a good agreement in volumetric efficiency and pressure fluctuations has been found. Finally, the leakage flow in the clearance disk has been analyzed.