Body fat calculator
Two ways to estimate the share of your bodyweight that is fat: one from your BMI and age, one from a tape measure.
Method
Sex
Units
Your body fat
18.5%
of bodyweight
Deurenberg BMI formula
1. Two methods, one number
The BMI method works from height, weight and age alone. The tape method, the US Navy circumference formula, works from your shape: neck and waist, plus hips for women.
Whichever you pick, stay with it. The two formulas read different signals, so a 20 percent tape reading and a 20 percent BMI reading are two different measurements that happen to share a number.
2. The BMI method (Deurenberg)
Deurenberg's 1991 formula estimates body fat from BMI, age and sex:
- Body fat %
- 1.2 × BMI + 0.23 × age - 10.8 × sex - 5.4
- sex
- 1 for men, 0 for women
It inherits BMI's blind spot: it cannot tell muscle from fat, so it overestimates lean, muscular builds. Use it when there is no tape within reach, and switch to the tape when there is.
3. The tape method (US Navy)
The Navy formula estimates body fat from circumferences. Measure your neck just below the Adam's apple with the tape level, and your waist at the navel after a relaxed breath out. Women also measure hips at the widest point.
- Men
- 495 ÷ (1.0324 - 0.19077 × log₁₀(waist - neck) + 0.15456 × log₁₀(height)) - 450
- Women
- 495 ÷ (1.29579 - 0.35004 × log₁₀(waist + hips - neck) + 0.221 × log₁₀(height)) - 450
These are the centimetre forms of the equations, the same coefficients the app runs. The imperial toggle converts your inches to centimetres before the formula, so both unit systems return the same number from the same tape.
4. Reading the number
Both methods are estimates with a few percentage points of give. Measure at the same time of day under the same conditions, and read the trend across weeks rather than any single result.
Lower is not automatically better. Below roughly 5 percent for men and 13 percent for women you are into essential fat, the minimum your body needs to function, and health and performance both suffer there.
5. The same math in the app
BDYCTRL runs both methods inside the measurement log and saves the result to your body fat history, beside your weight and circumference measurements. Get the app.