The U.S. Navy method
This calculator uses the circumference method developed by Hodgdon and Beckett at the U.S. Naval Health Research Center and still used by the U.S. military. It estimates body fat from height and a few tape measurements, which makes it free and repeatable at home. For men the formula, with measurements in centimeters, is 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450. For women it is 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450.
For example, a man who is 178 cm tall with a 38 cm neck and an 88 cm waist comes out at about 18.7% body fat. US measurements in inches are converted to centimeters first, so either unit system gives the same answer.
How to measure correctly
Use a flexible, non-stretch tape and measure on bare skin, keeping the tape level and snug without compressing the skin. Measure the neck just below the larynx (Adam’s apple), angling the tape slightly downward toward the front. Men measure the waist horizontally at the level of the navel; women measure at the narrowest point of the natural waist. Women also measure the hips at their widest point. Stand relaxed, breathe out normally, and do not suck in your stomach. Take each measurement two or three times and use the average.
Measure at the same time of day under the same conditions, ideally in the morning before eating. Small changes in posture or how tight the tape is can shift the result by a percent or two.
What the categories mean
The results use the American Council on Exercise ranges. For men, 2 to 5% is essential fat, 6 to 13% athletic, 14 to 17% fitness, 18 to 24% average and 25% or more obese. For women the ranges are higher because of essential fat linked to reproductive health: 10 to 13% essential, 14 to 20% athletic, 21 to 24% fitness, 25 to 31% average and 32% or more obese. Body fat that is too low can be as harmful as too much, affecting hormones, bone density and immune function.
Accuracy compared with other methods
Circumference methods typically land within 3 to 4 percentage points of laboratory techniques such as DEXA scans or hydrostatic weighing for most people. They are less accurate for very lean, very muscular or very large bodies, and for people whose fat is stored unusually. Bioelectrical impedance scales are convenient but vary with hydration. Skinfold calipers can be accurate in experienced hands. Whatever method you use, the trend over time is more meaningful than any single number, so measure every few weeks rather than daily. Your result also shows fat mass and lean mass, which helps you see whether a weight change came from fat or muscle.
Frequently asked questions
How accurate is the Navy body fat formula?
For most adults it is within about 3 to 4 percentage points of DEXA. It is less reliable for very muscular, very lean or very large people.
Why do women need a hip measurement?
Women tend to store more fat on the hips and thighs, so the women’s formula includes hip circumference to capture it.
What is a healthy body fat percentage?
Roughly 14 to 24% for men and 21 to 31% for women covers the fitness and average ranges used by the American Council on Exercise.
Why does my result look wrong?
Check that the waist is measured at the right point and that the tape is level. A neck measurement that is too large or a waist that is too small will make the result unrealistically low.