Siri Body Fat Equation Calculator

About this calculator

The Siri equation, published by physicist William Siri in 1961, converts a person's body density into body fat percentage using a two-component model of the body: fat mass, with a density of about 0.9 g/cm³, and fat-free mass (muscle, bone, water, organs), with a density of about 1.1 g/cm³. The formula is body fat % = (495 ÷ body density) − 450, where body density is usually measured by hydrostatic (underwater) weighing or air-displacement plethysmography (a Bod Pod).

Exercise physiologists, sports scientists and researchers use the Siri equation because it is more accurate than skinfold or bioelectrical-impedance estimates when precise body density is available, though it assumes constant tissue densities that can vary slightly by age, sex and ethnicity — which is why the related Brozek equation is sometimes used as a cross-check.

This calculator takes your measured body density and body weight and returns your body fat percentage along with your fat mass and lean (fat-free) mass in absolute units, using the classic Siri formula.