How to Measure Body Fat With Hydrostatic Underwater Weighing

Underwater Hydrostatic Body Fat Weighing
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Hydrostatic underwater weighing, or hydrostatic testing, is a method of determining body composition (the ratio of body fat to lean mass). It measures a person's total body density using Archimedes' principle of displacement.

Hydrostatic underwater weighing has long been considered the gold standard for body composition assessment. However, new and more sophisticated methods may make underwater weighing obsolete in the near future.


Underwater weighing is based upon 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. We can use this principle to determine percentage of body fat because the density of fat mass and fat-free mass are constant. Lean tissue, such as bone and muscle, is denser than water, and fat tissue is less dense than water.

Because muscle sinks and fat floats, a person with more body fat will weigh less underwater and be more buoyant. Someone with more muscle, meanwhile, will weigh more underwater.

Hydrostatic Weighing

Hydrostatic weighing is an underwater body composition assessment where a person is first weighed on dry land and then the person gets into a large tank of water. While sitting on a special scale, they are lowered underwater and asked to expel all the air from their 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 the 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.


This method of body composition analysis is considered to be very accurate and is used to judge whether other methods are accurate. It usually is consistent and so it can be used to measure progress. However, the person administering the test needs to understand the principles and do the calculations correctly.

A source of error in the test is the need to get all air out the subject's lungs when weighing underwater, as any air remaining will make them float and throw off the results. This is one reason that three underwater weights are taken.

Where to Get a Hydrostatic Underwater Weighing Test

Many universities offer hydrostatic weight tests for research purposes, and several companies offer testing for the public. Some, such as Body Fat Test, provide a mobile hydrostatic body composition testing service. The lack of easy access to this testing makes is less than ideal to use, especially since other methods of measurement are widely available.

Other Forms of Body Fat Testing

Hydrostatic weighing is considered to be accurate, but there are other technologies being developed that may replace it. One is the Bod Pod, which uses air displacement in much the same way hydrostatic weighing uses water displacement. Another is dual energy X-ray absorptiometry (DEXA), which measures bone mineral content as well as estimating body fat and muscle.

Bioelectric impedance body fat monitors are widely available for home use and at health clubs. They use a low voltage electric current to assess body composition. However, these monitors are less accurate than hydrostatic weighing and dependent on several variables. Using fat calipers is another accessible, easy method.

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By Elizabeth Quinn, MS
Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics.