Application of the Kozeny-Carman equation for estimating the porosity of filter cakes formed in filtration is examined. It is shown that the Kozeny-Carman equation belongs to a class of relationships relating the pressure gradient and flow rate for fluid flow through porous media and is applicable to unconsolidated randomly packed media composed of spherical or nearly spherical particles. In contrast, filter cakes formed under pressure may be compressible and are not homogeneous. Furthermore, the question whether filter cakes may be considered as unconsolidated remains unresolved. These complicating factors plus the fact that the calculated porosity values based on the Kozeny-Carman equation tend to be exceedingly low as compared with those obtained from independent measurements strongly suggest that using the Kozeny-Carman equation for determining cake porosity is questionable and not appropriate. (C) 2012 Published by Elsevier B.V.