The permeability of dust cakes on a filter for both air and water was studied theoretically and experimentally. The study investigated, in particular, the effects of particle polydispersity and shape factor on the permeability. In the experimental study, both air and water permeability tests were carried out on six different types of powders including monodisperse spheres and various polydisperse non-spherical particles. Using the parameters derived from a theory that took into account the effects of the particle polydispersity and particle shape, all the collected data were found to correlate well with the dust cake porosity ranging from 0.3 to 0.8 for both air and water permeation. The results suggest that the well-known Kozeny-Carman equation is not applicable for fluid permeation through particle layers having wide powder size distributions or irregular shape. With the proposed correlation, the permeation resistance of gas and liquid across powder layers consisting of non-spherical and polydisperse particles can be quantified. The correlation is useful for the design of filtration devices and the determination of operating conditions in filtration processes for both dust aerosols and colloidal suspensions.