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The Best Method to Calculate Body Fat

Excess body fat is an important risk factor for diseases such as hypertension, diabetes, respiratory dysfunction and obesity. Therefore, body fat percentage is an important indicator of your health and fitness. Hydrostatic/underwater weighing, considered the "gold standard," is accurate, reliable and one of the most widely adopted methods of body fat calculation.



Because fat is less dense than water, fatter people are more buoyant than leaner ones. Nonfatty parts of the body are denser than water, so the leaner a person is, the more they tend to sink rather than float. Hydrostatic weighing is a method to calculate body mass and composition that uses "The Principle of Archimedes." Archimedes' principle states that the weight of the fluid displaced when an object is immersed in water is equivalent to the amount of weight the object loses in water.

Things You'll Need

  • Regular weighing scale
  • Chair
  • Hydrostatic weighing scale
  • Underwater weighing tank
  • Helium analyzer
  • Computer
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Instructions

  1. Hydrostatic Weighing Method

    • 1

      Measure the dry weight of the individual using a regular weighing scale, and record the readings.

    • 2

      Place a chair on top of the hydrostatic weighing scale inside an underwater weighing tank, and adjust the scale to zero to subtract the weight of the chair. Then, instruct the person being weighed to sit on the chair.

    • 3

      Ask the person being weighed to exhale as much air as possible out of the lungs before submerging. Calculate the residual lung volume (RV) of the individual using a helium analyzer.

      A helium analyzer is a spirometer that employs the helium dilution technique to measure lung capacity as well as residual volume after exhalation. In this technique, helium gas is added to a closed system within a spirometer. Helium is an inert, odorless, colorless, tasteless and nontoxic gas that cannot cross the alveolar membrane and remains inside the lungs. Residual volume, or RV, is the air present inside the small air pockets of the lungs after maximum exhalation. This must be accounted for to obtain an accurate estimation of body composition.

      Alternatively, RV can be estimated based on the age, weight and gender of the individual.

    • 4

      After determining RV, instruct the subject to slowly bend forward, immersing the head completely, and stay still. Within 10 seconds, the scale inside the tank will weigh the submerged person and transmit the reading to a computer.

    • 5

      Perform the weighing six to 10 times. Take an average of the two or three highest results for the underwater weight value.

    • 6

      Calculate the individual's body density using body weight, RV and underwater weight using the following equation:

      Body density = dry weight of the individual / [(dry weight -- underwater weight / water density) -- RV -- 0.1]

    • 7

      Calculate body fat percentage from the total body density using either the formula developed by Siri or Brozek.

      Siri formula -- percent Fat = [(495 / Body density) -- 450] * 100

      Brozek formula -- percent Fat = [(4.570 / Body density) -- 4.142] * 100

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