Siri Body Fat Percentage Calculator

How it is calculated

The Siri equation turns a measured body density into a fat percentage: BF% = 495 / D − 450, where D is body density in g/cm³ (kg/L). Fat mass is W × BF% / 100 and lean mass is W − fat mass, with W the body weight in kilograms. Both density and weight are required. The tool accepts a density above 0.9 and up to 1.2, but gives no result if the computed body fat is below 0% or above 75%, which in practice means a density from about 0.943 to just under 1.1. It assumes fat and lean tissue have fixed densities, so the density itself must come from underwater weighing or an air-displacement chamber, not from a scale.

Common mistakes

• Typing a density in kg/m³ (1060) or as a ratio from a different test. The field expects g/cm³, around 1.03 to 1.09 for most adults, and 1060 returns no result.
• Entering body weight in the density field or the other way round. A value like 80 in the density field is outside the 0.9 to 1.2 window and is rejected.
• Using a density at or above 1.1. That would mean zero or negative fat, so the tool returns nothing instead of a number.
• Forgetting that weight is needed. Without it the percentage alone is not shown, because fat and lean mass cannot be computed.
• Comparing the result with skinfold or BMI-based estimates as if they were the same measurement. Siri converts a density, so its accuracy depends entirely on how well the density was measured.