We test the relation k= (m2/k0)F-1.5, where k is permeability, m is hydraulic radius, F is the formation factor, and k0 is a constant close to 2.5. Parameters k, m, and F have been measured in the laboratory or taken from the literature for two granites and a number of sandstones and ceramics. Predicted and measured k agree to within about a factor of 2 over the 9 orders of magnitude range of k. A similar relation closely predicts the change in permeability with stress measured for granite. Application to rocks at depth seems limited at present, for although deep resistivity may be known, information on the size of pore spaces, needed to calculate m, is generally not available.