Estimating weight on a standard scale does not always give one 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 percentage of fat to lean muscle. Hydrostatic weighing, 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.

Those undergoing hydrostatic weighing generally sit in a chair, attached to a scale, in warm water that is slightly under their chin. Persons being weighed often wear a heavy jacket to prevent floating, which can affect test results. Those being weighed may breathe through a snorkel so the amount of air in the lungs can be weighed and subtracted from the total weight.

During a hydrostatic weighing, the person will be instructed to completely submerge his or her head into the water. At this point, the person may be asked to blow the air from his or her lungs and hold his or her breath for about five seconds. The process from start to finish takes about 20 to 30 minutes, and includes instructions prior to the hydrostatic weighing as well as outfitting the person being weighed. After deducting the air in the lungs and the heavy jacket, the result is a “true” weight and body composition.

These percentages are compared to the Body Mass Index (BMI) to see if the person’s weight is in a healthy range. Adult men have a BMI acceptable range of between 12-22%. Women have a slightly higher BMI, and anything between 18-28% is considered acceptable.

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 when they undergo hydrostatic weighing, 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 their BMI does not fall within a normal range. However, 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 true weight and BMI. 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. Skin fold test results are not quite as accurate as hydrostatic weighing, but they can be a good first step in determining BMI.