Internally toothed gears are used in planetary gearsets, a type of mechanical power transmission with high transmission ratios and a very compact design. The MATLAB scrip that acts as the basis for the results that have been obtained, was already capable of analysing the static transmission error, load sharing factor and contact pressures of a set of spur or helical gears. Starting from this software, the necessary modifications have been performed to achieve the same study for the meshing of an internal, or ring gear, with the planet of the epicyclic gearing. Particular attention has been aimed at correctly determining the contact pressures generated by the gear meshing.