In modeling the effect of transcranial direct current stimulation, the representation of the skull is an important factor. In a spherical model, we compared a realistic skull modeling approach, in which the skull consisted of three isotropic layers, to anisotropic and isotropic single-layer approximations. We simulated direct current stimulation for a range of conductivity values and investigated differences in the resulting current densities. Our results demonstrate that both approximation methods perform well, provided that the optimal conductivity values are used. We found that for both the anisotropic and the isotropic approximations the optimal conductivity values are largely dictated by the equivalent radial conductivity of the three-layered skull.