The Kozeny equation is commonly used to determine the rate of filtration. This equation is derived from the Poiseuille equation, which states that the rate of filtration has a fourth-order dependency upon the pore radius. The Kozeny equation uses the Poiseuille equation with an average pore size determined based upon porosity and particle surface area. Because the pore sizes vary widely depending upon the particle size distribution, the use of an average pore size can lead to significant errors in determining the rate of filtration for samples with large changes in size distribution and small changes in surface area, In this study the Poiseuille equation was used together with the pore size distribution, which was calculated based upon particle contact probabilities, in order to more accurately determine the effect of size distribution on the filtration rate. Artificial particle size distributions of silica beads and gypsum particles were used at relatively low pressures to demonstrate the usefulness of the proposed model.