Rotary valve air leakage is an important parameter for the design and operation of many dilute- and dense-phase pneumatic conveying systems. Previous methods to estimate air leakage have been largely empirical and have been found inaccurate and/or limited in their use. This paper describes a new theoretical model that has been developed to estimate rotary valve (radial clearance) air leakage. Model predictions are compared with experimental data obtained on a rotating and stationary valve subjected to a wide range of operating pressures typical of dilute- and dense-phase systems. Knowing accurately the air leakage present for any given situation, the required total supplied air mass flow rate can be determined properly for the pneumatic conveying system, so that optimal transport velocities can be established.