This paper presents a model, obtained through numerical simulation and curve fitting, for the dielectric effective permittivity of frequency selective surfaces (FSSs). The numerical model is based on the permittivity and thickness of the dielectric layer supporting the FSS metallic elements, as well as on the period and on a factor dependent on the FSS geometry. The model is developed for square-loop FSS, using a given geometry reported in literature as a reference case. The results from electromagnetic simulations with the finite-element method are used for developing the numerical model.