Using data from the literature, the following simple empirical equation is derived for the cumulative mass fraction of particles broken (m) when impacted at a specific impact energy (J/g of impacted mass) of E:[GRAPHICS]In this equation, i is an integer index of a root2 screen size interval of upper size x(i); x(o) is a standard size of 1 mm; E-min, is the minimum specific impact energy required to produce significant breakage of feed material of size i and E-max is the specific impact energy that breaks all feed material of size i. (A) over bar, C and m are material-dependent constants. Values of (A) over bar = 0.235, m = 0.58 and C = 0. 13 J/g were found for particles of limestone in a thin bed impacted by a falling steel ball. The equation shows that smaller particles require a higher specific impact energy than larger particles to give the same fraction of breakage out of the feed size interval. (C) 2003 Elsevier Science B.V. All rights reserved.