Siri Body Fat Percentage Calculator

About this calculator

The Siri equation, developed by William Siri in 1961, converts a measured body density into body-fat percentage using the formula Body Fat % = (495 / Density) − 450. Body density itself is typically measured through hydrostatic (underwater) weighing or air-displacement plethysmography (a Bod Pod), both of which estimate density from a person's weight and volume — techniques that require lab equipment rather than a bathroom scale.

Because it's derived from a two-compartment model (assuming the body is only fat mass and lean mass with fixed densities), the Siri equation remains a reference standard that other, simpler body-fat estimation methods are validated against. This calculator takes your measured body density and returns your body-fat percentage along with your resulting fat mass and lean mass in weight.