Hydrostatic or Underwater Weighing Procedure

The most accurate means to calculate body fat percentage is with hydrostatic or underwater weighing. This method gives you a measurement of your true body composition.  This procedure is difficult because it requires precise equipment such as a pool or large tub of water and well trained technicians to execute. This test is based on the premise that fat floats and muscles sinks. The weight that registers on the scale is plugged into a formula to find your results. All weights are measured in kilograms but you may convert it into pounds after you plugged kilograms into the formula.

How the test is preformed

1.  You are your subject sit on a scale in a tank or large body of water. It would be large enough to submerge your entire body.

2. Blow all of the air out of your lungs then bend forward until you’re completely submerged. It is important to make sure that your lungs are empty for an accurate reading.

3. Stay underwater for five seconds while the digital scale weights your body.

4. Once you get your results in kg’s, plug this into the formulas below. If you want to know your weight in pounds, simple convert it after you plug in the numbers.

Siri and Brozek formulas


Determination of Body Density 

Body Density = dry weight / [((dry weight - wet weight) / water density)- RV - 0.1]  

Note: Units for all weights are in kg and RV is in L. The 0.1 represents an estimated volume (L) of gas in the GI tract.

Estimation of Percent Fat

The two most commonly used equations for estimating percent fat from body density are the Siri (1961) and Brozek (1963) formulae. A limitation to these formulae is that they assume the density of fat-free mass to remain a constant across the population when in fact is varies. Thus, the actual percent fat tend to be slightly higher than the measured percent fat in the lean, muscular individual and the opposite effect in obese individuals.


Percent Fat = [(495 / Body Density) -450] * 100


Percent Fat = [(4.570 / Body Density) - 4.142] * 100


1 Kg = to 2.20462 pounds 

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