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Underwater weighing testing

The purpose of underwater weighing (hydrostatic weighing) is to measure the volume of the body, using a water displacement method [based on Archimedes’ principle]. From this, and the person’s weight in air, the density of the body can be calculated [density = mass / volume]. The person undergoing the test is weighed before entering a hydrostatic weighing tank, and is then lowered into the tank, expelling all air from their lungs, until they are completely submerged and weighed again. Given some further assumptions about what two components the human body is made up of, that is, fat mass and fat-free mass, the percentage body fat can then be estimated. Essentially a fat person will weight a lot less in water because fat mass is less dense than fat-free mass. This is why fat people float better.