Hydrostatic Weighing

Hydrostatic weighing, also called underwater weighing or hydrodensitometry, is a method for estimating body composition from body density. It is based on Archimedes' principle: a submerged body experiences an upward buoyant force equal to the weight of the water it displaces. By comparing a person's weight in air with their apparent weight while submerged, the volume of water displaced and therefore body volume can be estimated .¹

Once body mass and body volume are known, body density is calculated as mass divided by volume. During conventional testing, the person is submerged and typically exhales as much air as possible before an underwater weight is recorded. Air remaining in the lungs contributes to buoyancy, so residual lung volume must be measured or estimated and accounted for when calculating body volume .^1,2

Hydrostatic weighing does not directly measure body fat. Traditionally, body density is converted into estimates of fat mass and fat-free mass using a two-compartment model that assumes characteristic densities for these compartments. Because the composition and density of fat-free mass can vary between individuals and populations, deviations from these assumptions can introduce error into estimates of body fat percentage .^2,3

Hydrostatic weighing is therefore best understood as a densitometry method: body mass and volume are used to determine whole-body density, from which body composition is estimated .^1,2