Body fat is necessary up to a point, but monitoring the body fat (BF) percentage change is important for diet and exercise programs. Water displacement, or hydrodensitometry, is usually accepted as the "gold standard" for BF percent measurement. But other devices and calculations might be preferable depending on which resources, needs, estimates and assumptions are acceptable to the user.
Water and Air Displacement
Researchers at Wright State University's School of Medicine in Ohio have compared methods and report that both the water and air displacement methods for measuring BF percent are the most accurate systems available.
The water displacement method measures body weight while totally submerged, after exhaling fully. The water volume displaced is also measured, minus the residual air volume in the lungs, as first determined on land by a pulmonary function test. Accounting for the less-than-water density of fat, and the ratio of submerged weight to displaced volume, accurate BF percentage is then calculated.
The similar procedure, body plethysmography, displacing air instead of water, has been found equally accurate.
But none of this can be done at home, so "correct" technique might depend on what is possible.
Impedance Devices
Dr. Edward Laskowski, writing for the Mayo Clinic, says most commercially available body fat analyzers---specially modified electrical impedance meters---are not highly accurate. Results can vary widely because of differences between instruments and the state of a person's hydration when tested. However, their portability and ease of use make them popular, even if they're not always accurate.
Radiology
A DXA unit (dual energy X-ray absorptiometry) available in hospital or clinic radiology departments reliably and accurately measures BF percentage. DXA distinguishes between fat, muscle, bone and other body components by the way each absorbs different spectra of X-rays. Though not the "gold standard," DXA is considered suitable for general use. Older literature might refer to this method as "DEXA."
U.S. Navy Calculations
Formulas reported in 1984 by researchers J. Hodgson and M. Beckett at the U.S Naval Research Center in San Diego can provide consistent results for people who prefer to exercise brain power instead of buying power---to calculate instead of paying for tests and meters.
For men, assuming the more typical male fat distribution---the apple shape:
Percent Fat=495/(1.0324-0.19077(log(waist-neck)+0.15456(log(height)-450
For women, assuming the more typical female fat distribution---the pear shape:
Percent Fat=495/(1.29579-0.35004(log(waist+hip-neck)+0.22100(log(height)-450
A "log" is a common base 10 logarithm. Logs are numbers looked up on standard mathematical tables or a mathematical or scientific calculator.
Body Mass Index Calculations
Dr. Steven Halls at the University of Alberta, Canada, says BF percentage calculated from body mass index (BMI), using only height, weight, age and sex, miscalculate for people with a lot of muscle mass, which this method cannot distinguish from body fat. The results for the same person, over time, can show useful trends, but comparing one person to another can be misleading.
Many different formulas have been developed by researchers. Selecting the best one for different individuals might require consultation with a physician or professional trainer.
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