A hydrostatic body fat test, known as hydrodensitometry,  is calculating percentage of body fat through water displacement.

More Info: A hydrostatic body fat test utilizes Archimedes principle, which states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object. 

Archimedes Principle

In Syracuse, Sicily c 250 BC, Archimedes, a Greek mathematician, inventor, and philosopher invented the compound pulley, the hydraulic screw, and is credited with defining the principle of the lever.  Archimedes became most famous for discovering the law of hydrostatics referred to as the Archimedes principle.

Applying Archimedes Principle to Body Fat Measurement

Fat is less dense than water and therefore floats.  The greater the fat percentage of an individual, the less he will weigh when submerged in water. Muscle and bone are denser than water and will therefore weigh more.

How Is a Hydrostatic Body Fat Test Performed

The subject is first weighed in the normal manner. His lung capacity will be measured using a spirometer.  He will then sit on a special chair that has a scale attached.  The chair is slowly immersed in water.  The subject must exhale all the air from his lungs, as this will cause buoyancy and a false reading.  Finally, when the entire subject is immersed and underwater weight reading will be taken.