The efficiency of a flat filter was changed by varying its thickness by laminating it into multiple layers. The response of pressure drop with mass challenge and mass accumulation, both per filter face area, were compared by challenging the filter with monodisperse, solid particles, loading the fillers well past filter cake formation. For successive layers of filter medium, the mass challenge data resulted in a family of curves, while the mass accumulation data did not. This showed that, in this case in which filter structure was uniform with depth, clogging took place in the front portion of the filter structure. The effective pore diameter, calculated from the Biter effective fiber diameter and solidity, was shown to give a confident linear correlation to the mass challenge per filler face area al filter clogging. This correlation was independent of particle diameter for effective pore diameters of 8.8-75 mu m when loaded with monodisperse particles of diameter 1-4 mu m.