Hydrostatic underwater weighing

Hydrostatic underwater weighing is a form of densitometry (another being air displacement plethysmography), which derives body composition from body density and body volume. It uses Archimedes' principle of displacement.

It is based upon the classic two-component (2-C) model of body composition which assumes that body weight is composed of fat free mass (FFM) with a constant density of 1.10 kg/L, and fat mass (FM) with a constant density of 0.90 kg/L. The density of the whole body, therefore, depends upon the relative size of these two components.

As bone and muscle are denser than water, a person with a larger percentage of fat free mass will weigh more in the water, and have a lower percent body fat. Conversely, fat is less dense than water. Therefore, a large amount of fat mass will make the body lighter in the water and have a higher percent body fat.

Body density is expressed as mass per unit volume, where mass is the weight of the body in air (Ma) and volume is the difference between the weight in air and the weight of the body submersed during underwater weighing (Ma – Mw). Once body density is calculated, it is possible to convert this to a percentage body fat.

The process is divided into three steps: 1) measurement of residual volume; 2) measurement of dry bodyweight; and 3) measurement of underwater weight. Figure 1 provides an illustration of one method used for hydrostatic underwater weighing.

Two methods to determine total body water

An allowance must be made for residual lung volume (the air that remains in the lungs after a maximal exhalation) as this air increases the participant's buoyancy. It is measured using methods such as closed circuit dilution (including helium dilution) or nitrogen washout. If indirect measurement cannot be conducted, it can also be estimated from vital lung capacity or using published equations that are based on age, height.

Hydrostatic underwater weighing typically requires the participant to be completely submerged underwater while exhaling maximally to minimize the effect of buoyancy from lung air. Differences in residual volume determination have been reported to contribute the largest sources of variation.

Dry bodyweight

After having measured or estimated residual lung volume, the participant's dry weight is measured wearing a swimming suit or running shorts.

Underwater weight

  1. The participant climbs into the underwater weighing tank and sits in a seat that hangs from a force load-cell.
  2. The procedure involves the participant performing a maximal exhalation and slowly leaning forward in the chair until the top of his/her head is under water.
  3. After the underwater weight stabilizes (approximately 5 seconds), the participant is instructed, though yelling or pounding on the side of the tank, to return to an upright position.
  4. During weighing, the participant is always free to stand up or return to the upright position.
  5. Several trials may be required before the participant becomes accustomed to the procedure and consistent readings recorded. Four to eight trials are usually performed.
  6. The depth of the water is approximately 140cm, and temperature maintained between 30 and 34 C. The density of water is dependent on the temperature and should always be factored into the equation. After each testing session, the water is drained and the tank allowed to dry.

Pre-test Guidelines

Participants must not eat or engage in strenuous exercise for at least 4 hours before their scheduled appointment. They should avoid ingesting any gas-producing beverages for at least 12 hours before the test. Participant must wear swim suits.

If each test is performed correctly according to the recommended guidelines, hydrostatic underwater weighing has small percentage error (+/- 1.5% error), and has been labelled the gold standard assessment of fat mass/fat free mass. However, as it is mostly only available in research laboratories or university settings, it is generally used for research purposes and not applicable in clinical practice or in large-scale population studies.

It is used less frequently since the introduction of air displacement plethysmography as this technique is better tolerated by participants.

Figure 1 Illustration of measurement of underwater body weight.
Source: http://nutritionfirstfitness.com

Body density is required to calculate percentage body fat. It is calculated using the following:

  1. Participant’s weight outside water (Ma)
  2. Participant’s weight entirely submerged in water (Mw)
  3. Density of water in which participant is immersed (Dw)
  4. Residual volume (estimated or actual) (RV)

The substitution of these values in the given formula estimate the body density:

Body density = Ma / [(Ma – Mw) / Dw – RV]

Once body density has been calculated from the data obtained, body composition can be estimated. The most commonly used equations for estimating the percent of body fat from density are as follows:

Siri Percent fat = [(495 / body density) - 450] * 100

Brozek Percent fat = [(4.570 / body density) - 4.142] * 100

An overview of the characteristics of hydrostatic underwater weighing is outlined in Table 1.


  1. It is a reliable and accurate technique


  1. Time intensive
  2. Labour intensive
  3. Considerable participant cooperation required
  4. Participant discomfort
  5. Inaccessible for some populations such as elderly, disabled and chronically ill
  6. Requires specially trained professionals to setup and conduct the testing procedure
  7. Participant’s hydration level may affect the error in test results significantly
  8. Being submerged underwater might be a little difficult for some (e.g. elderly)
  9. Participant must be able to perform complete air exhalation
  10. There is a possibility of considerable error introduced by the miscalculation of residual volume (RV)
  11. The test does not identify the exact parts of the body where the fat is located (e.g. unable to assess fat distribution)
  12. The accuracy of body density measurements can be affected by the consumption of food and carbonated beverages shortly before the test, fluid loss during intensive exercise, fluid retention during menstruation, and the degree of forcible exhalation while submerged
  13. Like all the 2 component model methods, it assumes when calculating total body density that the relative amounts and densities of bone, muscle, and water comprising the fat-free mass are essentially the same for all individuals, regardless of age, sex, ethnicity or fitness level

Table 1 Characteristics of hydrostatic underwater weighing.

Consideration Comment
Number of participants Small
Relative cost Medium
Participant burden High
Researcher burden of data collection High
Researcher burden of coding and data analysis Low
Risk of reactivity bias No
Risk of recall bias No
Risk of social desirability bias No
Risk of observer bias No
Space required High
Availability Medium
Suitability for field use No
Participant literacy required No
Cognitively demanding No

Considerations relating to the use of hydrostatic underwater weighing for anthropometry in specific populations are described in Table 2.

Table 2 Anthrompometry by hydrostatic underwater weighing in different populations.

Population Comment
Pregnancy Suitable but unable to disentangle the mother-fetal unit
Infancy and lactation Not suitable
Toddlers and young children Not suitable
Adolescents Not suitable
Older adults May not be suitable as older individuals have less dense bones, resulting in over-estimation of body fat
Ethnic groups Suitable
Other (athletes) Athletes tend to have denser bone and muscle tissue, resulting in under-estimation of body fat
Other (obesity) Suitable
  1. Hydrostatic stainless steel weighing tank
  2. Underwater mounted chair and scale
  3. Weighted belt and nose clip
  4. A simpler set up may include a chair and scale suspended from a diving board over a pool
  5. Trained research staff
A method specific instrument library is being developed for this section. In the meantime, please refer to the overall instrument library page by clicking here to open in a new page.
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