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In the design of pleated air filters, the velocity of the flow at the filter surface is many times approximated as the “effective” velocity computed by assuming that the air flow is evenly distributed over a cross-sectional area equal to that of the “unfolded” filter. In our findings this effective velocity is much lower (by a factor of 10 to 40) than the average “duct” velocity that would be computed two to four centimeters upstream of the filter plane. This effective velocity factor would depend on parameters such as type of filter media (fiber diameter, filter solidity and material) and pleat configuration (height and spacing). When used to design engine intake or cabin air filters, the effective velocity may be misleading if used in a predictive theory for filter efficiency. In reality, the upstream portion of the pleat may be exposed to a considerably higher velocity than the downstream portion of the pleat.