The pressure drop generated by a fixed weight of fibrous filter material increases with decreasing filament size and with increasing specific surface area. To be generally useful, a pressure drop model must account for both of these effects. Because of the complexity of solving flow equations for irregularly shaped elements, most flow field terms are derived for cylindrical fibers, and an adjustment to the radius must be made for the non-round cross-sections typically used in cigarette filters. Although the equivalent-area radius correctly accounts for the filament size, it underestimates the surface area of the fiber, and therefore underestimates the pressure drop. The equivalent-surface-area radius overestimates the size of the fiber, and therefore overestimates the pressure drop. A comparison of predicted and observed pressure drops for a wide range of"Y"cross-section tow items indicates that the optimum radius to use in a pressure drop model corresponds to the maximum arm extension for"Y"cross-section filaments. The maximum arm extension, then, is the effective aerodynamic radius of the filaments, and accounts for the effects of both surface area and filament denier on pressure drop. To evaluate the generality of this approach, aerodynamic radii were estimated for a variety of cross-sectional shapes, and the corresponding predicted pressure drops were found to agree well with measured values.