A method for estimating an air mass flow (m) entering a cylinder of a four-stroke positive ignition engine (E), equipped with an intake manifold (1M) and at least one intake valve and a pressure (PS) and a temperature (TS) sensor associated with the intake manifold, the method being based on a model for estimating the air mass flow rate comprising a first contribution ( f ( p,T,N e ,η,.,. ) ) calculated by means of a modelling in stationary conditions of the estimate of the mass of air entering the engine, a second contribution m p , T , … p ˙ p μ function of an average pressure and a relative derivative over time, estimated as a function of pressure at the intake manifold a third contribution m p , T , … T ˙ T υ as a function of a temperature and a relative time derivative estimated at said intake manifold, wherein the second contribution is positive, while the third contribution is negative.