Using the traditional filtration-rate equation and the modified permeation theory that takes into account the effect of particle polydispersity and particle shape, the physical meaning of the specific cake resistance is considered theoretically. As a result, the specific resistance is expressed as a function of particle shape, mean particle size, polydispersity, cake porosity and particle density. The equation is useful for the filtration calculation as an alternative to the Kozeny-Carman equation. For non-spherical particles, with a wide size distribution, the effects of particle shape and polydispersity on the specific resistance are not negligible. The effect of cake porosity on the specific resistance is discussed on the basis of previously obtained experimental results.