Underwater Weighing - Hydrostatic Weight

Definition: Underwater weighing, or hydrostatic weighting is a method of determining body composition or the various components that make up a person's total body density using Archimedes' Principle of displacement. Underwater weighing has been considered the gold standard for body composition assessment, however new, more sophisticated methods may make underwater weighing obsolete in the near future.

Underwater weighing is based upon the assertion that the density of fat mass and fat-free mass are constant, lean tissue such as bone and muscle are more dense than water, and fat tissue is less dense than water. Therefore, a person with more body fat will weigh less underwater and be more buoyant. Someone with more muscle will weigh more underwater.

To perform underwater weighing, a person is first weighted on land. Next the individual will get into a large tank of water. While sitting on a special scale, he is lowered underwater and asked to expel all the air from his lings and remain motionless while the underwater weight is measured. This procedure is repeated three times and averaged.

A special calculation is then used to determine lean weight and fat weight and determine a person's percentage of body fat.

See a Video of underwater weighing procedure from the University of Vermont.

Also Known As: Body Composition Measurement , Hydrostatic Weighing, Body Fat Measurements, Body Composition, Body Density

Examples: The Archimedes principle states that when a body is submerged in water, there is a buoyant counter force equal to the weight of the water which is displaced.