hydrostatic weighing

(fluid mechanics) A method of determining the density of a sample in which the sample is weighed in air, and then weighed in a liquid of known density; the volume of the sample is equal to the loss of weight in the liquid divided by the density of the liquid.

Weighing a person fully submerged in water. The difference between the person's mass in air and in water is used to calculate body density and then from this estimate the proportion of fat in the body. Although one of the most accurate methods of estimating body composition, hydrostatic weighing is usually not very practical or convenient. See also bioelectrical impedance and skinfold measurements.

A technique used to estimate body volume. A person is weighed in air (normal scale weight) and then weighed when totally immersed in water (underwater weight). The difference between the scale weight and underwater weight is used to calculate body volume. In the calculation, the density of water and the gas trapped in the lungs is taken into account. See also densitometry.