How the U.S. Navy method works
Body fat percentage is the share of your total weight that is fat. The most accurate ways to measure it (DEXA scans, underwater weighing, air-displacement pods) need a clinic or a lab. The U.S. Navy method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984, estimates it with nothing more than a tape measure. Fat tends to collect around the waist and hips while the neck changes much less, so the gap between those circumferences, scaled by your height, tracks body fat surprisingly well.
The calculator uses the metric (centimetre) form of the equations. If you enter inches they are converted to centimetres first.
Men: %BF = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
Women: %BF = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
Worked examples
| Person | Measurements | Body fat |
|---|---|---|
| Man (the default values) | height 178 cm, neck 38 cm, waist 86 cm | 17.2% |
| Same man, waist 95 cm | height 178 cm, neck 38 cm, waist 95 cm | 23.6% |
| Woman | height 165 cm, neck 33 cm, waist 74 cm, hip 100 cm | 28.9% |
| Man (imperial) | height 70 in, neck 15 in, waist 34 in | 17.4% |
The second row shows why the waist matters so much: 9 cm more around the middle, with everything else unchanged, adds about six percentage points. If you also enter your weight, the calculator splits it into fat mass and lean mass (everything else: muscle, bone, organs and water). For the first man at 80 kg, 17.2% body fat is about 13.8 kg of fat and 66.2 kg of lean mass.
The Navy also publishes an inch-based version of the formula with different constants. It gives almost the same answer (17.5% instead of 17.4% for the imperial example), so do not worry if another site differs by a few tenths of a point.
What is a healthy body fat percentage?
There is no single official cut-off, but the categories published by the American Council on Exercise (ACE) are widely used:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25% and above | 32% and above |
Women carry more essential fat than men because of breast tissue and fat needed for hormones and reproduction, so the same number means something different for each sex. Going below the essential range is unhealthy: it can disturb hormones, menstrual cycles, immunity and bone health. Body fat also rises gradually with age in most people, so a 55-year-old in the "average" band may be in good shape for their age.
How to measure correctly
- Use a flexible, non-stretch tape and measure on bare skin in the morning, before eating.
- Keep the tape level and snug but not compressing the skin.
- Neck: just below the larynx, with the tape sloping slightly down towards the front. Do not flare the neck.
- Waist (men): horizontally at the level of the navel, at the end of a normal breath out. Do not suck in.
- Waist (women): at the narrowest point of the abdomen, usually a little above the navel.
- Hip (women): at the widest part of the buttocks, feet together.
- Measure each site two or three times and use the average. Rounding to the nearest 0.5 cm (or ¼ inch) is enough.
How accurate is it?
Compared with laboratory methods, tape-based equations like this one usually land within about 3 to 4 percentage points for most adults. They are less accurate for very muscular people with thick necks (who may be underestimated), for people whose fat is stored mostly on the hips and thighs, and for children, pregnant women and older adults with low muscle mass. The real strength of the method is tracking change: if you measure the same way every few weeks, a falling number is a reliable sign you are losing fat, even when the scale barely moves because you are also gaining muscle.
Body fat, BMI and waist size
BMI only uses weight and height, so it cannot tell muscle from fat: a muscular rugby player and an inactive person of the same height and weight get the same BMI. Body fat percentage separates the two. Waist size on its own is also a strong health signal because fat around the abdomen is linked to type 2 diabetes and heart disease; the WHO flags increased risk above a waist of 94 cm for men and 80 cm for women. Using all three together gives a much clearer picture than any one number. To set a target weight, try the ideal weight calculator, and to plan how much to eat, the calorie calculator.
Changing your body fat
Fat loss comes from a sustained calorie deficit, usually a modest one of about 300–500 kcal per day, combined with enough protein and some form of resistance training so that the weight you lose is mostly fat rather than muscle. Expect the tape to move slowly: losing 0.5–1% of body fat per month is a realistic, sustainable pace for most people. Re-measure every two to four weeks under the same conditions rather than every day.
This calculator gives an estimate for general information. It is not a medical diagnosis; speak to a doctor or registered dietitian for personal advice.
Frequently asked questions
How do I calculate body fat with a tape measure?
Measure your height, neck and waist (and hips for women), then use the U.S. Navy formula. For a man 178 cm tall with a 38 cm neck and 86 cm waist, the result is about 17.2% body fat.
What is a good body fat percentage?
Using the ACE categories, 14–17% is 'fitness' and 18–24% 'average' for men; for women, 21–24% is 'fitness' and 25–31% 'average'. Obesity starts at 25% for men and 32% for women.
How accurate is the U.S. Navy body fat method?
It is usually within about 3–4 percentage points of laboratory methods for most adults, and it is very good for tracking change over time if you measure the same way each time.
Why do women need the hip measurement?
Women store more fat on the hips and thighs, so the women's formula adds the hip circumference to the waist to capture it.
Sources
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center, Report No. 84-11 and 84-29, 1984.
- American Council on Exercise (ACE). Percent body fat norms for men and women.
- World Health Organization. Waist circumference and waist–hip ratio: report of a WHO expert consultation, 2008.
Last reviewed 14 October 2026. Results are estimates; see our disclaimer.