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What is Hydrostatic Weighing?

Tricia Christensen
By
Updated Mar 03, 2024
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Estimating weight on a standard scale does not always give an accurate measurement of body health. Since a high percentage of body fat can significantly affect health, hydrostatic weighing can be used to measure not in pounds but in body density. This method, also called underwater weighing, involves being weighed underwater. It is not a particularly complex form of weighing, but it does require one to be completely submerged in water for a few seconds. Those who have a significant fear of being completely underwater should probably forgo this test.

Before the person is weighed under water, he is usually weighed on land. Measurements of lung volume should also be taken so that accurate calculations can be made during the test. Mistakes in these measurements can lead to inaccurate test results.

Those undergoing hydrostatic weighing generally sit in a chair, attached to a scale, in warm water that is slightly under their chin. The person is then asked to exhale as much air as possible and to completely submerge his or her head into the water for a few seconds. The process from start to finish takes about 20 to 30 minutes, and includes instructions prior to the weighing as well as outfitting the person being weighed.

After deducting the residual air in the lungs, the result is the subjects weight under water. This is entered into a calculation which also takes into account the weight taken on land and other factors to determine body density. This figure can then be used to determine the person's body fat percentage.

Those who try hydrostatic weighing are often surprised, either pleasantly or negatively, by the results. Trained athletes often have higher than average standard weight than non-athletes, but they often have quite low body fat counts. Muscle weighs more than fat.

Conversely, sometimes thin people have a higher than normal fat percentage. These people are generally surprised if body density does not fall within a normal range. This information can be helpful in planning a healthy diet and exercise regimen that reduces body fat content and raises lean muscle mass.

Some health clubs and dieting organizations offer hydrostatic weighing. There are also clinics specifically for hydrostatic weighing and nutritional advice. Not many doctors offer this technology, although they may refer a patient to a specialist or clinic offering the test. Often, treatment centers for anorexia and other eating disorders have hydrostatic weighing equipment, as it can help focus patients on weight not derived from a scale and may cause less concern for patients who need to be weighed.

Hydrostatic weighing is currently considered the best method for measuring body density, and thus body fat. It is, however, expensive, and infrequently covered by health insurance. Should one be unable to afford this type of weighing, one can ask doctors about the noninvasive and quick skin fold test. Most doctors can perform this upon a patient’s request.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a The Health Board contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
Discussion Comments
By anon94192 — On Jul 07, 2010

For an accurate estimation of body fat the Bod Pod, as Denisecaton indicated, is an alternative to hydrostatic weighing and produces similar results, but skinfolds done accurately are just as good assuming you use a valid equation.

If you want an accurate estimation of body fat, the best thing to do is call your local University and contact their Exercise Physiology Department and they can test you.

Anon26390, there are several ways to fool a hydrostatic weighing test, but not if done correctly. If you want to show a higher body fat simply blow out less air when under water.

It is all about density, how much space you take up versus how much you weigh. The more you weigh and the less space (volume) you take up the more dense you are and the more muscle you have. Obviously, the more dense you are the less likely you are to float, if you are mostly fat then it is easy to float.

By denisecaton — On May 20, 2009

There is also a method for accurately testing body fat called the Bod Pod. Same concept as far as density measurement, but does not require the underwater experience. The Bod Pod measures air displacement, so it's completely non-invasive.

By anon26390 — On Feb 12, 2009

Is it possible to cheat the hydrostatic weighing to show more of less body fat percentage? If so, how?

Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a The Health Board contributor, Tricia...
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