A method to calculate electrostatic forces in the context of atomic force microscopy that is useful for the calculation of the electrostatic forces when different length scales are included in the simulation is presented. The versatility of the method allowed for an analysis of the behavior of forces as a function of the tip apex geometry. For example, for flattened, worn out tips, the force at the point of contact with a dielectric sample could be 2.5 times larger than that of a sharp tip. A simple analytical approximation has been also developed for the local characterization of thin films at the nanoscale.