The g-values of some selected low-spin Fe(I) and Fe(III) porphyrin complexes have been calculated, using a method based on density functional theory and the zeroth order regular approximation to the Dirac equation. In agreement with experimental observations the calculated g-values of these complexes differ strongly from the free electron g(e)-value. Optimization of the geometries gives iron-ligand distances in good agreement with the experimentally derived values. The effects of strong ruffling of the porphyrin core and of the relative orientation of the planes of axial ligands on the g-values are evaluated quantitatively. Mechanisms reported for these effects are confirmed.