Although many factors control the oxidation rate of a metal, the Pilling–Bedworth ratio is a parameter that can be used to predict the extent to which oxidation may occur. The Pilling – Bedworth ratio is Md/nmD , where M and D are the molecular weight and density, respectively, of the corrosion product scale that forms on the metal surface during oxidation; m and d are the atomic weight and density, respectively, of the metal, and n is the number of metal atoms in a molecular formula of scale; for example, for Al₂O₃, n = 2.