In this paper, we present a phenomenological model that predicts the size distribution of the product issued from impact crushing in function of the rotor strike radius and velocity, the material properties and size distribution of the feed as well as the feed rate. The model is based on the standard matrix representation including classification and breakage matrices. It can be applied to both horizontal- and vertical-shad impact crushers by means of the corresponding estimations for the average impact energy per unit mass presented here. We propose a new classification function for impact crushers in the form of a Weibull cumulative distribution. The minimum, size of the particles that undergo breakage is assumed to be a function of the impact energy and the feed rate. The model predictions are compared with experimental data obtained for limestone treated in a pilot-plant hammer crusher. The dependence of the product size distribution on the rotor velocity is investigated. The influence of the feed rate on the product size is also simulated. (C) 2004 Published by Elsevier B.V.