An algorithm for the calculation of the electrostatic potential distribution governed by the Poisson-Boltzmann equation is presented. The algorithm is applicable to both a single planar surface and two parallel, identical, planar surfaces. The surface under consideration is coated with an ion-penetrable membrane, which bears fixed charges. Both uniformly distributed fixed charges and nonuniformly distributed fixed charges due to the dissociation of the functional groups in the membrane are considered. The liquid phase contains an arbitrary a:b electrolyte or mixed a:b and c:d electrolytes. The result obtained for two parallel surfaces is readily applicable to the estimation of both the electrostatic pressure and the interaction free energy between these surfaces.