Siri Body Fat Equation Calculator

How it is calculated

The tool applies the Siri equation to a measured body density: body fat % = 495 / density − 450, with density in g/cm³ (for example 1.05). Fat mass is weight × body fat % / 100 and lean mass is weight − fat mass, both in kilograms, so enter weight in kg. The density has to come from a measurement such as underwater weighing or air-displacement plethysmography; the tool cannot estimate it from height and weight. Only densities between 0.9 and 1.2 and weights between 2 and 650 kg are accepted; anything else gives no result. If the formula gives a value below 0, the result is clamped to 0 % (and to 100 % at the top end). Siri assumes fixed tissue densities, so individual results can differ from the real value by a few percentage points.

Common mistakes

• Entering body density in kg/m³ (1050) or as a percentage. The field expects g/cm³, a value near 1.0 to 1.1; anything outside 0.9 to 1.2 gives no result.
• Entering weight in pounds. Fat and lean mass are labelled in kg, so a weight in pounds gives wrong masses even though the percentage stays the same.
• Using a density from a bathroom scale or an estimate. The equation is only as good as the underwater or air-displacement measurement behind it.
• Skipping the correction for air in the lungs in underwater weighing. An uncorrected density comes out too low and the fat percentage too high.
• Reading a 0.00% result as real. Densities above about 1.10 give a formula value at or below zero (1.15 shows 0.00%), which the tool clamps to 0 %; it means the input is unrealistic, not that you have no fat.