An equation which represents the amount of damage to metallic materials by solid particle impact was derived by comparing the behavior under quasi-static and dynamic indentation. Both the diameter and the volume of the dynamic indentations formed by spherical or angular particles could be described by an exponential function of the material hardness (Hv), K(Hv)-N. It was found that the constant K and the exponent N in the equation depended upon factors which were related to the aggressiveness of the impact, such as impact velocity, impact angle, and the size and shape of the particles. The relationship between the erosion damage caused by 250μm silica sands and the surface hardness of the material after testing followed this equation for both pure metals and heat-treated alloys, over a wide range of material hardness (Hv 43-1100). The surface hardness gradually changed with the number of particle impacts, reaching a constant value with regular damage on the material surface. Work-hardening was observed in most metals. However, for heat-treated hard steel alloys, not only work-hardening but also softening were observed on the damaged surface.