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Brozek Body Fat Percentage Calculator

Turn body density into body-fat percentage with the Brozek equation.

How it is calculated

The Brozek equation converts body density into body fat: BF% = 457 / D − 414.2, where D is body density in g/cm³. It is the same relationship as the textbook form (4.57 / D − 4.142) × 100. Fat mass is W × BF% / 100 and lean mass is W − fat mass, with W the body weight in kilograms. Both inputs are required. The density must be above 0.9 and at most 1.2, and the tool returns nothing when the computed fat is under 0% or over 75%, which in practice means densities from about 0.935 up to about 1.103. At the same density it gives a slightly lower percentage than the Siri equation.

Worked example

A person weighs 80 kg and has a measured body density of 1.06 g/cm³. Divide 457 by 1.06 to get 431.13, then subtract 414.2. Body fat is 16.93%. Fat mass is 80 × 16.93 / 100 = 13.55 kg and lean mass is 80 − 13.55 = 66.45 kg. For comparison, the Siri equation on the same density gives 16.98%.

Try this example →

Common mistakes

  • Entering density in kg/m³ (1060) instead of g/cm³ (1.06). The tool rejects values above 1.2, so you get no result.
  • Using the percentage form of the formula and the engine form together. The constants 457 and 414.2 already include the factor of 100, so do not multiply the output again.
  • Mixing results from Brozek and Siri when tracking progress. They differ by a few tenths of a percentage point at the same density, so stick with one equation over time.
  • Typing a very high density such as 1.12. Body fat would be negative, so the tool returns nothing rather than a number.
  • Leaving weight blank and expecting at least the percentage. Weight is required, and without it the tool shows no result at all.

Sources

About this calculator

The Brozek equation converts a body density measurement — typically obtained via underwater (hydrostatic) weighing, air displacement plethysmography (Bod Pod), or skinfold-derived estimates — into a body-fat percentage, using the formula %BF = (4.57/density − 4.142) × 100. It's one of the two classic density-to-fat conversion formulas, alongside the Siri equation, and tends to give slightly different results because it assumes a different composition for fat-free mass.

This calculator takes your measured or estimated body density and returns your body-fat percentage along with the split between fat mass and lean mass in absolute terms. Exercise physiologists, sports scientists, and body-composition labs use the Brozek formula specifically because certain reference populations and research studies were standardized on it, making it the preferred conversion in some clinical and athletic settings.

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