Body Fat Percentage (estimate)

Background

This calculator estimates body fat percentage from BMI, age and sex using the Deurenberg equation: body fat % = (1.20 x BMI) + (0.23 x age) - (10.8 x sex) - 5.4, where sex is 1 for men and 0 for women. It was developed as a way of converting BMI into an approximate fatness figure when no direct measurement is available. It measures nothing about you directly — it is an educated conversion of your height, weight, age and sex.

Interpreting the result

Read the result as a rough band, not a number. Because the equation is driven by BMI, it inherits BMI's central weakness: it cannot distinguish muscle from fat. A heavily muscled person will be told they carry far more fat than they do, and a sedentary person with little muscle may be told they carry less. Reported standard errors for equations of this kind are in the region of 4 percentage points, which is large relative to the differences people care about. If the figure matters clinically, use a direct method — DEXA, air displacement plethysmography or skinfolds by a trained assessor.

Advice

This is an estimate only and can be several percent off for very muscular or very lean people. It is not a substitute for proper body-composition measurement.

Worked example

A 40-year-old woman, 68 kg and 165 cm. BMI = 68 / (1.65 x 1.65) = 24.98. Body fat = (1.20 x 24.98) + (0.23 x 40) - (10.8 x 0) - 5.4 = 29.98 + 9.2 - 5.4 = about 33.8%.

Pearls / pitfalls

• The equation cannot see muscle. Athletes and regular strength trainers will usually be overestimated, sometimes substantially.
• Because the only inputs are height, weight, age and sex, two people with identical figures get identical results regardless of how differently they are built.
• The relationship between BMI and body fat varies by ethnicity, so the equation carries additional error in populations unlike the one it was derived in.
• For tracking change over time, waist measurements and how clothes fit are usually more informative than a repeatedly recalculated estimate from the same equation.

Evidence & validation

The equation comes from Deurenberg, Weststrate and Seidell (1991), who derived age- and sex-specific prediction formulas relating BMI to body fat measured by densitometry. It was developed in a general adult population and is known to be less reliable at the extremes of body composition and in populations that differ from the derivation sample, including some ethnic groups in whom the BMI-to-fatness relationship differs.

Frequently asked questions

References

1. Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr. 1991;65(2):105-114.

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