Body Density

Body density is the mass of the body divided by its total volume. In body-composition assessment, it is a derived quantity that can be used to estimate the relative amounts of fat mass and fat-free mass. Body volume can be measured using methods such as hydrostatic weighing or air-displacement plethysmography, after which body density is calculated from body mass and volume .^1,2

In the traditional two-compartment model, fat mass is assumed to have a density of about 0.90 g/cm³ and fat-free mass about 1.10 g/cm³. Within this model, a greater proportion of fat mass lowers whole-body density, while a greater proportion of fat-free mass raises it. Equations can use this relationship to convert body density into an estimate of body fat percentage .¹

Body density does not directly measure fat mass or any individual tissue. The two-compartment approach depends on assumptions about the density and composition of fat-free mass. Fat-free mass contains water, protein, minerals, and other components whose proportions can vary among individuals, causing its actual density to differ from the assumed value .^1,2

More detailed body-composition models can combine body density with measurements such as total body water and bone mineral content. By accounting for variation in these components, three- and four-compartment models reduce some of the assumptions required when body density alone is used to estimate body fat .^1,2