Archimedes was on to something. Hydrostatic weighing uses simple science to provide highly accurate results about your fat vs. lean mass.

It's been a few years, but you might remember a simple experiment on displacement from science class. When you add an item to a glass of water, as illustrated in Figure 1 below, the water level rises as the mass of the submerged item displaces the liquid. The concept of displacement is the secret behind hydrostatic weighing. This procedure is one of the most accurate methods of measuring body composition, determining the percentage of fat compared to lean mass (muscle and bone).

Only CT and MRI scans, which are used primarily for research purposes, can provide more accurate body composition measurements than hydrostatic weighing. In fact, scientists even use hydrostatic weight measurement to assess the accuracy of other body composition measurement methods. Many elite athletic training facilities have hydrostatic weighing capabilities. You may also find this service at universities, gyms, and medical centers. 

In this guide, you’ll get the answers to your hydrostatic weighing questions and explore the importance of understanding your body composition.

How does hydrostatic weighing work?

To make the leap from middle school lab to body fat calculation using hydrostatic weighing, we’ll look to the Archimedes principle. According to this fundamental fact, the force of water on an object immersed within water equals the weight of the displaced water.

Fun Fact: Archimedes isn't only known for his namesake principle. The ancient Greek philosopher and mathematician developed revolutionary screws and levers. He also helped define the concept of Pi and invented complex defense weapons like giant catapults.

With hydrostatic weighing, health care providers calculate the difference between your body weight on land and when immersed in a specialized water tank. Because fat weighs less than bone and muscle, this subtraction provides insight into body fat percentage. A higher weight in water than on land indicates a low percentage of body fat, while a lower weight indicates a higher fat density. 

Fun Fact: The difference in weight between body fat and lean mass explains why a person who has more body fat will float when submerged, while a muscular person will sink. This occurs because of the high density of muscle and bone and the low density of fat tissue compared to the density of water.

Each pound of muscle and fat has a constant mass and thus displaces a specific amount of water. The technician takes this measurement and uses a formula to assess body fat and lean muscle percentages. Body density equals Ma/[(Ma – Mw) / Dw – RV], with variables defined as:

• Ma for the subject's weight on land

• Mw for the subject's weight when submerged

• Dw for the water density of the tank

• Rv for the actual or estimated residual volume, which is the amount of leftover air that may remain in the lungs after you completely exhale

After calculating body density, the technician uses one of two common equations to determine body fat percentage using these numbers:

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

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

When you get your numbers, the technician provides insight into your fitness status. See the below charts to review the health of your body fat percentage so you can plan to maintain or strive to lose excess fat and build lean muscle mass. Everyone needs a certain level of essential body fat, but too much fat increases the risk of obesity-associated diseases, such as cancer, type 2 diabetes, high blood pressure, and heart disease.

What happens during hydrostatic weighing?

When you schedule your appointment, the facility will provide instructions for your session. Generally, you must avoid eating or drinking for at least 4 hours before the procedure and should wear a swimsuit when you arrive for hydrostatic weighing.

When you arrive at the facility, the technician will first record your weight on dry land. Next, you will sit on a special underwater seat suspended from a scale in a large water tank, usually about 4 to 5 feet deep. On the technician's cue, you will completely exhale as you bend over to go completely underwater for about 5 seconds. When the test administrator records the weight, they will signal you to resurface.

To ensure correct results, you'll repeat this process three to five times. Your final results will be the average of these calculations. The test can also give inaccurate results if you still have air in your lungs at the time of measurement. Consistent conditions and measurement support the precision of this test, so technician error can also affect accuracy. Hydrostatic weighing may be less effective for people who have a chronic illness or disability as well as the elderly.

Fun Fact: No need to worry about goosebumps when you have a hydrostatic weighing appointment. The water in the tank remains at a comfortable 86 to 92 degrees F.

What are the benefits of hydrostatic weighing?

First and foremost, this procedure is free of side effects and safety concerns. You will only be underwater for a few seconds at a time, and the technician will monitor you through the entire process. It's also incredibly fast and convenient. In most cases, clients are in and out of a hydrostatic weighing appointment in just 15 to 30 minutes.

Hydrostatic weighing can also provide body composition information for people who have pacemakers. Common body composition tests using electrical impedance are unsafe for these individuals. 

Serious athletes rely on hydrostatic weighing for performance tracking because of its exceptional accuracy. According to research published in the Indian Journal of Medical Research, hydrostatic weighing can calculate body fat with precision 1.8 to 2.8 percentage points above the accuracy of other measures.

Even fitness newbies can take advantage of hydrostatic weighing to track progress toward health and wellness goals. You don't have to be a professional bodybuilder to measure how well diet and exercise interventions are driving success over time with this modality.

For example, consider having a body fat measurement before you begin a new fitness regimen to get a baseline. Then, you can retest every month or two to track your progress and make adjustments to your nutrition and exercise plan as needed.

What are the potential drawbacks of hydrostatic weighing?

While hydrostatic weighing is appropriate for most individuals who want to learn more about their body composition, it does have some limitations. Because this method cannot discern different types of fat or segment the distribution of muscle and fat in different areas, it cannot detect important issues such as visceral fat that wraps around the internal organs and increases disease risk. People who need details about fat type and distribution should consider the DEXA scan, which distinguishes between fat and muscle in the gynoid, android, arm, and leg areas and indicates the amount of visceral fat in the body.

How does hydrostatic weighing stand up to the BOD POD? And Dexa?

The BOD POD is another advanced method of measuring body composition. While this test also relies on the Archimedes principle, it displaces air rather than water to calculate fat and muscle mass with a device called a plethysmograph. Some of the key advantages of body fat testing with BOD POD include:

• A completely non-invasive, safe, comfortable experience, free from the logistical challenges of hydrostatic weighing and the small amounts of radiation exposure associated with dual-energy X-ray absorptiometry (DXA), another body composition modality

• The convenience of screening in less than five minutes, including just two minutes spent inside the BOD POD device

• A high level of accuracy that allows precision tracking of metabolism and body composition changes over a defined period

• Secure online results, including fat-free mass and total body fat measurements, available within one business day

Compared to hydrostatic weighing, BOD POD testing is more convenient, more easily accessible, and faster. Researchers point to the higher level of accuracy of air displacement for this purpose compared to water displacement,  with a lower risk of logistical issues compared to the potential margin of error with the latter approach.

Unlike DEXA scans, hydrostatic weighing and BOD POD screening do not provide information about visceral fat. These cells collect around the abdominal organs and are associated with a dramatically increased risk for chronic illness. Patients who have concerns about health risks such as type 2 diabetes, hypertension, heart disease, and cancer should consider booking a DEXA scan in their area for information that can help reduce this elevated disease risk. 

During the test

First, the technician will note your weight and height. Then, you will enter the BOD POD device, which has two separate areas. Instead of entering a pool of water, you will sit on a seat in the front test chamber, separated from the rear reference chamber by a wall with a securely sealed door. A diaphragm on this wall adjusts the air pressure in the test chamber as the technician takes three to five weight measurements. He or she takes three to five measurements of the internal air volume of the chamber while you are inside. Each measurement takes about 45 seconds. You must stay still and silent during the test to avoid affecting the accuracy of the results.

When you're working toward wellness goals, understanding and tracking your body composition provides a true picture of your progress. Find a Fitnescity BOD POD location near you and start your journey to better health. 

References:

https://www.healthline.com/health/hydrostatic-weighing

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6366261/ 

https://www.verywellfit.com/what-is-hydrostatic-underwater-weighing-3120276 

https://www.measurement-toolkit.org/anthropometry/objective-methods/hydrostatic-underwater-weighing 

https://healthcare.utah.edu/wellness/services/fitness/testing/bod-pod.php