Body Fat Testing Through Underwater Weighing

Your body weight is only one piece of your overall health picture. Body composition, or how much fat vs lean mass you have, matters as well. There are several methods to measure an individual’s body composition. Knowing what percentage of your weight is fat and lean muscle mass can help you tailor diet and exercise to meet your unique health goals.

Underwater weighing is a traditional and reliable method of measuring body composition.

Underwater Weighing

Underwater weighing, also known as hydrostatic weighing, is a method of calculating body fat percentage based off body density. Underwater weighing takes place in specially designated pools and/or water tanks designed for hydrostatic measurement. The test is usually only available at universities, research facilities, and some fitness centers.

How does underwater weighing work?

Before the underwater weighing takes place, the individual being measured will be weighed outside the water. Then, wearing a bathing suit, the individual will enter the pool or tank and sit in a chair that is submerged in the water.

The chair will either be lowered further into the water or the individual will be asked to lower their head into the water until their entire body is submerged. Once submerged, the individual exhales as much air from their lungs as possible by blowing bubbles.

The test administrator records the weight when no more air bubbles appear in the water and the individual can then resurface. The test is typically repeated several times and the average of the weights used to minimize errors.

The recorded weight will be plugged into equations that account for any leftover air in the lungs and gastrointestinal tract to calculate body density in water and total body fat percentage.

How accurate are the results?

Underwater weighing is considered one of the most accurate forms of measuring body composition. Rate of error for underwater weighing is estimated at 1-3%.

Underwater weighing was once considered the “gold standard” for comparing accuracy and reliability of other body composition measurement methods. However, with newer technologies available, it’s being used less often since it does require special equipment.

What are optimal testing conditions?

The individual being weighed should be comfortable getting in and out of a pool and/or tank of water while wearing minimal clothing, such as a bathing suit.

The individual should also be capable and comfortable expelling air from their lungs and keeping still while submerged underwater.

Pros of Underwater Weighing:

  • Underwater weighing is highly accurate with one of the lowest margins of error.

Cons of the Underwater Weighing:

  • It’s not the ideal choice for everyone, since it requires fully submerging yourself underwater. Some people may not be comfortable getting into water or lowering their head beneath the surface.
  • The equipment used for underwater weighing isn’t widely available and/or accessible.
  • The test may be an out-of-pocket expense to the individual being weighed.

Conclusion

Underwater weighing is one method for calculating percentages of body fat and lean mass. Although highly accurate, it is not widely available, and therefore, may not be convenient for taking regular weight measurements.

Other methods, such as bioelectrical impedance or skinfold measurements, may be more convenient for tracking changes in weight and body composition.

Ana Reisdorf, MS, RD is a freelance health writer and dietitian with 12 years of experience in the field of nutrition.

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