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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.

Siri Equation Calculator

Enter the body density in this calculator.


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 individual) and fat-free mass (everything else: bone, water, lean tissue etc). Studies have determined that the densities 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.

Similar Equations

Here are a few other equations that similarly convert body density to percent body fat. Various equations are required to best fit different populations, age groups and gender.

Brozek Percent Fat Equation:
%fat = (457 / Body Density) – 414.2

Schutte Percent Fat Equation (for African American males):
%fat = (437.4 / Body Density) – 392.8

Wagner Percent Fat Equation (for African American males)
%fat = (486 / Body Density) – 439

Ortiz Percent Fat Equation (for African American females)
%fat = (483.2 / Body Density) - 436.9


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