Electric force microscopy is a local technique for measuring electrical properties of materials. The electrostatic force gradient measurements on dielectric samples are sensitive not only to the initial charge distribution in the sample but also to the charge induced by the conductive bias cantilever. Interpreting the contribution of each single effect on the charge distribution images is a challenge in the existing EFM technique. Here, a theoretical model is introduced to study the charge and induction effect on charged dielectric samples and commercial geometries for EFM tips. This model estimates the initial charge of the sample based on force gradient measurements. Gradient force results reproduce experimental measurements performed on electrets samples.