Estimating Body Fat Percentage

Estimates body fat percentage from height, weight, age and sex with the Deurenberg formula. It is an estimate, and it can differ from a measurement.

Explanation

Body fat percentage is the share of your weight that is fat. It can be estimated from height, weight, age and sex with the Deurenberg formula.

\text{body fat} = 1.2 \times \text{BMI} + 0.23 \times \text{age} - 10.8 \times S - 5.4

where $S$ is 1 for men and 0 for women.

Example

A 30-year-old man, 170 cm, 65 kg, giving a BMI of $65 \div 1.7^2 = 22.49$ .

1.2 \times 22.49 + 0.23 \times 30 - 10.8 - 5.4 = 26.99 + 6.9 - 16.2 = 17.7\ \%

His fat mass is $65 \times 0.177 = 11.5$ kg, leaving 53.5 kg of lean body mass: muscle, bone, organs and water.

Typical ranges

Men — 10 to 19% is normal, 20 to 24% is high, 25% and above is obese
Women — 20 to 29% is normal, 30% and above is obese

Women sitting higher is not a defect. Hormones and reproduction require a larger fat reserve.

Lean mass is what matters

A diet should shed fat, not muscle and bone.

The bathroom scale cannot tell the difference. Crash-dieting away 5 kg is no victory if half of it was muscle: you have simply lowered your metabolism and made the weight easier to regain. Watch the split between fat mass and lean mass, not the total.

Watch out

This is an estimate, and not a precise one — expect several percentage points of error.

Because it is built on BMI, it overstates the body fat of muscular people. The formula cannot tell whether a high BMI comes from muscle or from fat. Run an athlete through it and the answer will be far too high.

For a real number, use a body composition scale or a DEXA scan.