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Fitness Testing > Tests > Anthropometry > Body Composition > Siri Equation
Siri Equation
Many body composition equations derive their measure of percent body fat from first determining body density. Once body density is determined, percent bodyfat (%BF) can be calculated using the Siri equation below :
% Body Fat = (495 / Body Density) - 450.
Derivation
The Siri Equation is based on the two compartment model, that is the body is made up of essentially two components: fat mass (the total fat of an indiviual) and fat-free mass (everything else: bone, water, lean tissue etc). Studies have determined that the denisities of these two components are:
fat mass = 0.9 grams per cc
fat-free mass = 1.10 grams per cc.
As density = mass / volume, and the mass of a human is made up of the total of fat mass and fat-free mass, therefore density = (fat mass + fat-free mass) / volume. Following this through and substituting mass/density for volume, and using the values for density above, eventually you get to the Siri Equation as listed.
References
- Siri, W. E. (1961). Body composition from fluid space and density. In J. Brozek & A. Hanschel (Eds.), Techniques for measuring body composition (pp. 223-244). Washington, DC: National Academy of Science.
Similar Equation
Brozek Equation: Percent Fat = (4.57 / body density – 4.142) x 100
Related Pages
- About body density
- Calculating percent bodyfat
- Skinfold Measures
- A discussion about Body Composition Testing
