In this paper we develop a performance model for impact crushers. The product size distribution is obtained as a function of the crusher's rotor radius and angular velocity, the feed rate and the feed size distribution. The model is based on the standard matrix formulation that includes classification and breakage matrices. It can be applied to both hammer and vertical-axis impact crushers with the help of the corresponding estimations for the impact energy per unit mass. Here we propose classification and breakage functions for impact crushers taking into account the dynamic character of the impact breakage. The classification function has the form of a cumulative Weibull distribution and incorporates a minimum breakable size of the particles depending on the impact energy and the feed rate. The breakage function is modelled as the sum of two Broadbent-Callcott distributions. It is assumed to depend on the impact energy and the feed rate through the proposed expression for the proportion of the fine fraction in the product. The model predictions are compared with experimental data for limestone treated in a pilot-plant hammer crusher. The variations of the product size distribution resulting from changes in the rotor velocity and the feed rate are investigated. (C) 2002 Elsevier Science Ltd. All rights reserved.