The total flow of air through perforated paper consists of two components : viscous flow through the inherent porous structure, and inertial flow through the perforation holes. The total air flow (Q, cm3 min-1) is related to the pressure difference (P, kilopascal) across A cm2 of the paper by : Q = ZAP + Z'A(P)n where Z is permeability due to viscous flow (CORESTA units), Z' is permeability due to inertial flow, and n is a constant for a given set of perforation holes. This equation adequately describes gas flow through a variety of perforated cigarette and tipping papers. By using different gases, it is confirmed that Z depends on viscous forces and Z' depends on inertial forces. By examining the flow of air through a large number of papers with different sized perforation holes, it is shown that Z' is dependent on the total area of perforation holes, and that a jet contraction effect occurs as the air travels through the paper. The parameter n is shown to have a value between 0.5 and 1.0, and this value is related to mean perforation hole size. The permeability of cigarette paper is defined as the flow of air through the paper when the pressure across the paper is 1 kilopascal. Thus from the equation the"total permeability"of perforated cigarette paper is equal to (Z + Z').