This document specifies the determination of the overall specific external and internal surface area of either disperse (e.g. nano-powders) or porous, solids by measuring the amount of physically adsorbed gas according to the method of Brunauer, Emmett and Teller method,[1] based on the 2015 International Union for Pure and Applied Chemistry (IUPAC) recommendations[3]. NOTE For solids exhibiting a chemically heterogeneous surface, for example, metal-carrying catalyst, the BET method gives the overall surface area, whereas the metallic portion of the surface area can be measured by chemisorption methods.