 In electrochemical catalysis, parameters such as current, resistance, or capacitance must be referred to the electrochemically exposed area, otherwise they lack physical sense. For this reason it is necessary to know this exposed area as accurately as possible. Although there are several experimental methods to estimate the electrochemically exposed area, these are usually not suitable for different metals and present large errors in measurements, for example when two or more metals are mixed. Moreover, the technical difficulties of carrying out this type of measure mean that it is often necessary to resort to approximations with great error, such as the spherical approach, with the aim of having an approximate value of the area. In this work, Molecular Dynamics simulations were performed to study the morphology of graphene-supported nanoparticles. A method for the calculation of the electrochemically active surface area is proposed and tested for Pt, Au, and Pt/Au nanoalloys. These results are compared with existent experimental data and a model is proposed for the calculation of the electrochemically active area for supported nanoparticles of different sizes.