Hydrostatic weighing

Hydrostatic weighing (i.e., underwater weighing) is a method for measuring the mass density  of a body.

This method has the advantage of needing no volume information of the body (see the note about the residual volume below). The procedure is based on Archimedes' principle, using the following three measurable values: The weight of the body outside the water, the weight of the immersed body and the density of the water.

Hydrostatic weighing (i.e., underwater weighing) is a method for measuring the mass density  of a body.

Method

This method has the advantage of needing no volume information of the body (see the note about the residual volume below). The procedure is based on Archimedes' principle, using the following three measurable values: The weight of the body outside the water, the weight of the immersed body and the density of the water. Then the formula below can be solved for the density of the body:
(The formula was taken from the article buoyancy

Buoyancy

In physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....

 and still has to be solved for the density of the body.)

Application

From this density information body composition

Body composition

In physical fitness, body composition is used to describe the percentages of fat, bone and muscle in human bodies. Because muscular tissue takes up less space in our body than fat tissue, our body composition, as well as our weight, determines leanness...

 and percentage body fat may be estimated. This is occasionally used for assessment of obesity

Obesity

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...

. The residual volume in the lungs can add error if not measured directly. Hydrostatic weights gives body density, and percent body fat can be estimated using the Siri or Brozek formula.