In a tDCS model study, the accuracy of isotropic and anisotropic single-layer approximations to the actual three-layered skull is evaluated. For both approximation models, the average difference in brain current density with respect to the layered skull model are shown to be small. We conclude that both approximations can be used to accurately compute the current density in the brain, provided that the radial conductivity in the model matches the effective radial conductivity of the three-layered skull.