The Wankel rotary engine offers a greater power density than piston engines, but higher fuel consumption and higher hydrocarbon emissions, in large part due to poor gas sealing. This paper presents a modeling approach to evaluate the gas leakage of apex and corner seals in rotary engines. The apex seal is modeled as a deformable beam and its dynamics is coupled with the gas flows around the seal. It is shown that the main leakage mechanisms are: (1) corner seal clearance leakage, (2) leakage around the apex seal through the spark plug cavities, and (3) flank leakage at high speed. The side piece corner orifice and the trailing spark plug cavity also contribute to leakage, but to a lesser extent. Leakage through the seal–housing interface is negligible as the apex seal can conform to the distorted shape of the rotor housing.