Body composition is an important measure of health and fitness as well as a mechanism for assessing the risk for some diseases. Since body habitus, bone structure and muscle mass all affect weight, this number is rarely taken alone as an indicator of fitness. Body fat percentage, which can be calculated from either of two formulas, is much more accurate in providing information about composition.
History
Formulas designed to calculate body fat have their roots in experiments done by W. E. Siri in the 1950s. He used density---determined by dividing a subject's weight by the volume of water he displaced in an immersion tank---to estimate body fat percentage. The theory behind the experimental design was that fat, which is less dense than lean tissue, would decrease the density of a subject and provide a correlation for determining the subject's lean mass and fat mass.
Types
While the logistics of water-immersion density calculations are prohibitive for most individuals, thankfully advancements in the Siri formula have produced two equations that allow for calculation of body fat from much simpler measures. The skinfold formula uses subdermal fat to approximate total body fat, while the circumference formula uses combined subdermal and abdominal fat to make the same approximation.
Skinfold Formula
According to researchers J. V. Durnin, Ph.D. and M. M. Rahaman, Ph.D., you can calculate body fat percentage by taking skinfold thickness measurements (in millimeters) at four sites: biceps over the midpoint of the muscle; triceps over the midpoint of the muscle; subscapular area below the tip of the lower scapular angle; and suprailiac just over the iliac crest. Calculate the log of the sum of the measurements, and use this value to estimate density by using the equation density = 1.1610 -- 0.0632 x (log of sum of measurements). Then calculate body fat percentage from density by the equation body fat percentage = [(4.95/density) -- 4.5] x 100.
Circumference Formula for Men
Hugo Rivera, an engineer and trainer, offers a formula for calculating body fat from circumferential measurements. Multiply body weight by 1.082, and add 94.42. Multiply waist circumference (taken at the belly button) by 4.15. Subtract the second number from the first number. This represents lean body weight. Subtract lean body weight from total body weight to calculate body fat. Divide body fat by total body weight and multiply by 100 to obtain percent body fat.
Circumference Formula for Women
Rivera modifies the calculation somewhat for women, whom he suggests will need additional circumference measurements to obtain an accurate result. Multiply body weight by 0.732 and add 8.987 (this is A in the formulas below). Divide wrist circumference (taken at the widest point) by 3.14 (this is B). Multiply waist circumference (taken at the belly button) by 0.157 (this is C). Multiply hip circumference (taken at the widest point) by 0.249 (this is D). Multiply forearm circumference (taken at the widest point) by 0.434 (this is E). Calculate A + B -- C -- D + E. This is lean body weight. Subtract lean body weight from total body weight to calculate body fat. Divide body fat by total body weight and multiply by 100 to obtain percent body fat.
References
- "Advances in Biological and Medical Physics"; W. B. Siri; 1956
- "British Journal of Nutrition"; The Assessment of the Amount of Fat in the human Body From Measurements of Skinfold Thickness; J. V. G. A. Durnin and M. M. Rahaman; 1967
- Dave Draper.com: Formula for Body Fat
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