A simple and general mathematical model has been developed to calculate filter-filtration efficiency, which is based on practical design parameters : pressure drop, filter dimension, flow rate, and filament denier. This model has been verified by examining published experimental data, which include a broad range of design parameters. This model surpasses other models currently described in the literature in terms of accuracy. The drag force of the filter (which is accounted for by the pressure drop times the circumference squared,DELTA P. sq.C) appears to be the most important contribution to filter-filtration efficiency, but its contribution can be drastically reduced by varying design parameters.