A method for determining a cylinder air charge for an internal combustion engine includes determining engine operating parameters including an intake air charge density, an engine delta pressure as a ratio of an intake manifold pressure to an exhaust pressure, and an engine speed. An open-throttle volumetric efficiency corresponding to the engine operating parameters is determined, and a closed-throttle volumetric efficiency corresponding to the engine operating parameters is also determined. The cylinder air charge is determined using a selected one of the open-throttle volumetric efficiency and the closed-throttle volumetric efficiency.