How to take the measurements
Use a flexible tape held flat against the skin, without pulling it tight, and enter everything in centimeters. Measure your neck. Men measure the abdomen at the navel. Women measure the waist at its narrowest point and the hips at their widest. The hip box is ignored for men. Weight is used only to split your result into fat and lean mass.
How the number is worked out
The U.S. Navy method, published by Hodgdon and Beckett in 1984, predicts body density from circumferences and height. In metric form, men use 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450. Women use 495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450.
Fat mass is weight × body fat percentage, and lean mass is what remains.
Worked example: a man at 178 cm
A man is 178 cm tall and weighs 78 kg, with a 38 cm neck and an 88 cm waist. Waist minus neck is 50, and the formula gives 18.7% body fat. That is 14.6 kg of fat mass and 63.4 kg of lean mass, in the Average category.
What your body fat category means
The tool labels your result with the American Council on Exercise categories below. They describe fitness groups and are not health diagnoses.
- Essential fat: men 2 to 5%, women 10 to 13%
- Athletes: men 6 to 13%, women 14 to 20%
- Fitness: men 14 to 17%, women 21 to 24%
- Average: men 18 to 24%, women 25 to 31%
- Obese: men 25% and up, women 32% and up
How far to trust a tape-measure estimate
Tape methods are estimates. In a 2022 study of fit U.S. Marines compared with DXA scans, this method read about 2.5 points low for men and 1 to 2 points high for women on average, overestimating lean people and underestimating those with higher body fat. Individual errors can be larger, and Marines are not the general public.
It was developed with adult Navy personnel, so it does not suit children or pregnancy, and bloating or a loose tape can shift the result.