1.There is a nonlinear relationship between structure and functional processes and the mass of living organisms. Allometric scaling of body mass provides a theory based method for predictions of mass related biological characteristics.2.Numerous empirical methods have been proposed to link the mass of living organisms with the size which is the driver for structure and function.3.Normal fat mass is a size descriptor based on allometric theory that partitions total body mass into fat free mass and fat mass. The relative influence of fat mass as a predictor of size compared to fat free mass is described by the fraction of fat mass that makes fat equivalent to fat free mass in terms of allometric size. This fraction will differ for each drug and each parameter affected by body size (e.g. clearance and volume of distribution).4.The use of normal fat mass based on allometric theory provides a principle based approach linking size and body composition to structure and function.5.Normal fat mass can be used to improve drug dosing. Display omitted