The weight of a person who is completely immersed in water can be used to calculate the density of the body which is the best available method for estimating the body fat content in a living person. This process is known by various terms including underwater weighing, body densitometry and hydrostatic weighing.

The principle of the measurement is simple and dates back to antiquity when Archimedes, a Greek scientist who lived more than 2,000 years ago, famously calculated the density of a king’s crown in order to determine if it was pure gold or not. Legend has it that Archimedes realized the solution to the problem while observing the water displaced by people as they entered the public bathing pools. From his observations he concluded:
1. A body displaces a volume of water equal to the volume of the body, and
2. The decrease in the weight of a body when immersed in water is equal to the weight of the displaced water.

In modern times, measurement of the body density of living persons by underwater weighing gained wide acceptance after the publication of two (JAMA) papers in 1942 by Dr. Albert Behnke et al. They showed that, in a large group of naval men and professional athletes who were classified as "overweight" for their height, the excess weight resulted from muscle rather than fat. Unfortunately, individuals deemed "overweight" were classified as unfit for military service and did not qualify for first class insurance rates. They concluded that rejections for military service on the basis of "overweight" status might serve to eliminate outstanding athletes.

Unfortunately, the widespread use today of the "body mass index" (BMI), which is based solely on height and weight, suffers from the same lack of validity as an indicator of body fatness. With proper equipment, underwater weighing provides a simple, safe and valid way to measure body density for the accurate estimation of body fat fraction (of weight). Virtually all other methods for estimating body fat percent, such as skin folds, body impedance analyzers, ultrasound, air displacement plethysmography and whole body X-ray scanners, derive their validity by comparing the results to body density data from underwater weighing.