This paper explains the procedure used to determine the amount of air leakage expelled from two different sized drop-through rotary valves. When designing a pneumatic conveying system, rotary valve air leakage may be overlooked, believed to be only a minor factor. This is somewhat true when conveying in dilute-phase where low pressures are generated, however in dense-phase pneumatic conveying where higher pressures can be produced, rotary valve air leakage can be a high proportion of the supplied air. Knowing the air leakage present for any given situation, the total supplied air mass flowrate can be adjusted to represent the actual air mass flowrate travelling through the conveying line, thus giving a more accurate representation of the actual conveying conditions. A series of tests have been performed from which a series of graphs have been produced. Comparisons have been made for the air leakage through the different sized rotary valves and also these results have been compared to two existing models used in the prediction of rotary valve air leakage.