Hydrostatic weighing, also known as underwater weighing or hydrodensitometry, is one of the most accurate ways to measure body fat.

It was considered the gold standard for measuring body composition until more advanced methods, such as CT scans and MRI scans, were developed.

During the test, you have your body weight measured on land and underwater. The difference between these two measurements allows test administrators to calculate your body density and body fat percentage.

Hydrostatic weighing is mostly performed on athletes, in research, or in other situations where an accurate measurement is needed.

Even though it isn’t commonly performed on most people, some universities, training centers, and gyms do offer testing. Some companies even offer mobile tests, for which the test administrator drives to your location.

Keep reading to find out how hydrostatic weighing works and how it compares to other body fat testing methods.

Hydrostatic underwater weighing is based on the Archimedes’ principle. This principle states that the amount of liquid a submerged object displaces is equivalent to the volume of the object.

By measuring your weight on land and your weight underwater, test administrators can calculate your body’s density.

Fat is lighter than bone and muscle, so your density can be used to calculate your body fat percentage. If your underwater weight is relatively heavy, it indicates a lower body fat percentage, and vice versa.

A simplified example would be measuring the density of a brick.

Imagine you have a brick that weighs 5 kilograms (about 11 pounds) on land and 4 kilograms (about 8.8 pounds) when measured on an underwater scale. This difference in weights means the brick displaces 1 kilogram of water, or the equivalent of 1 liter (about 0.3 gallons) of water.

From this, you can conclude that the brick has a volume of 1 liter and a density of 5 kilograms per liter.

Hydrostatic weighing procedure

Prior to the hydrostatic weighing, you’ll likely be instructed to avoid exercising or eating for at least 4 hours beforehand and to bring a lightweight bathing suit.

Here’s what to expect during the procedure:

  1. The test administrator measures and records your body weight on land.
  2. You then slowly enter a water tank that has an underwater seat hanging from a scale.
  3. Your testing administrator instructs you to completely submerge underwater while sitting on the chair and to breathe out fully.
  4. You repeat the test three times.

The administrator will use the average result of the three tests to calculate your body fat percentage.

Some of the advantages of underwater weighing compared to other body fat tests include the following:

  • It’s more accurate than other widely available methods of body composition testing.
  • The entire test can usually be performed in about 15 minutes.
  • Unlike electrical impedance body fat tests, hydrostatic weighing can be used safely on people with pacemakers.
  • The test is safe and doesn’t have any side effects.

Hydrostatic body fat testing is primarily performed in situations that require a highly accurate measurement.

Some athletes may use it to measure their body fat at different points during their season to monitor their progress. For example, they may take a baseline reading at the beginning of the training year and retest at the end of each training block.

Hydrostatic body fat testing is also commonly used in research to obtain an accurate measurement of body fat.

Although hydrostatic body fat testing isn’t commonly performed on most people, some gyms and universities offer tests for a fee.

Hydrostatic weighing is one of the most accurate ways to measure body fat. In fact, it was once considered the most accurate method to assess body composition.

Nowadays, MRI and CT scans are considered the gold standard, but their use is mostly limited to medical research.

When performed properly, hydrostatic weighing can be accurate to 1.8 to 2.8 percent compared to more advanced methods.

Skinfold calipers

Skinfold caliper testing is one of the simplest ways to measure body fat. There are several variations to the test, but often a three-site or seven-site test is used.

During the test, a test administrator uses a pair of calipers to measure the thickness of skin folds and the fat underneath at three or seven different locations on the body.

The administrator then uses those measurements to estimate overall body fat percentage.

Skinfold tests can be accurate to about 3 to 4 percent for 70 percent of the population, but they can be significantly less accurate when administered by somebody inexperienced.

Body fat scales

Bioelectrical impedance analysis is a common feature on bathroom scales. It uses a small electrical current to estimate body fat based on how easily the current moves through the body.

Fat is a poor conductor of electricity, so more resistance indicates a higher body fat percentage.

The accuracy of these scales can vary depending on the brand. Your level of hydration can also significantly change the results because water conducts electricity easily.

Commercially available scales are generally less accurate than hydrostatic weighing and other medical research tools, but they can provide you with a rough estimate of your body composition.

Dual-energy X-ray absorptiometry (DEXA or DXA)

DEXA uses X-rays to measure body fat, muscle mass, and bone mineral density. It’s the gold standard for measuring bone mineral density. It’s not quite as accurate for measuring body fat as hydrostatic weighing, however.

Some companies offer DEXA scans for about the same price or slightly more than hydrostatic weighing. For example, University of California, Davis offers DEXA scans for $50 after an initial assessment that costs $75.

MRI and CT scans

MRI and CT scans are the most accurate methods of measuring body fat. However, these imaging tests are usually limited to medical research and are very expensive.

The machines that perform MRI or CT scans work by taking multiple cross-section images of your body. A trained technician can identify various types of tissues, such as bone, muscle, or fat, in these images.

Hydrostatic weighing tests are available at some universities, medical research centers, and fitness centers.

If you want to get tested, you can try contacting universities or colleges in your area that have kinesiology or exercise science programs. You can also search for gyms or fitness centers in your area that may offer the test.

Some companies also offer mobile hydrostatic testing. This involves driving the hydrostatic tank to your location.

Hydrostatic weighing is one of the most accurate ways to measure body fat.

During the test, you’ll be submerged in water while you sit on a scale. An administrator will calculate your body fat percentage by comparing your land weight with your underwater weight.

This method of measuring body fat is most commonly used in research and with athletes, but some gyms and universities in your area may offer testing for a fee.