Hydrostatic body fat testing, also referred to as “underwater weighing”, “hydrostatic body composition analysis”, and “hydrodensitometry” is a technique for measuring the mass per unit volume of a living person’s body. It is a direct application of Archimedes principle, that an object displaces its own volume of water.
To perform underwater weighing, a person is first weighed on dry land. Next, the person 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 lungs 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. By volume, fat weighs less than muscle, and pound for pound, fat, and muscle each have a constant mass and displace a specific amount of water.
Referenced from Wikipedia and verywellfit.com