The formation of dust cakes on the surface of a flat square polyester felt has been studied. Cake porosity was estimated from pressure drop and deposited mass measurements, utilizing correlations from the literature. These results have been compared to direct porosity measurements, obtained by image analysis of cake cross sections. Cakes of limestone particles with four different size ranges (median Sauter diameters of 2.5, 3.3, 6.9, and 10.7 mu m) and four thicknesses, ranging from 130 to 1200 mu m, were tested. The results have shown cake porosity to be increasing considerably with decreasing particle diameter. Also, the porosity was found to be dependent on cake thickness and position, indicating compressible dust layers. The porosity estimated from the correlations agreed reasonably well with the measured average in a cross section and could be used in practical cases in which overall values are required, provided that the cake is not too thin. Finally, an expression for pressure drop in a compressible cake porosity, based on a differential form of the Ergum equation, was developed with good agreement with experimental results.