How the body fat calculator works
Choose Male or Female and Metric (cm) or Imperial (in), then enter your height and the tape measurements. Men need neck and waist; women need neck, waist and hips. Your body fat percentage appears on the scale as you type. Add your weight, which is optional, and you also get fat mass and lean mass. A body fat estimate goes a step further than BMI, which only compares weight with height; if you have not checked it yet, our BMI calculator shows both side by side with the WHO cut-offs.
The calculator uses the US Navy circumference method, developed by James Hodgdon and Mitchell Beckett at the Naval Health Research Center in 1984 and later used across the US Department of Defense. The equations work in inches (centimetres are converted by dividing by 2.54):
- Men: body fat % = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
- Women: body fat % = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
The idea is simple: the waist and hips grow with fat, the neck mostly with muscle and frame, and height scales the result.
How to measure
Tap a field and the matching line lights up on the figure. Use a soft tape and open How to measure, step by step in the tool for the full checklist:
- Neck: just below the Adam's apple, with the tape sloping slightly down towards the front.
- Waist (men): horizontally at the level of the navel, at the end of a normal breath out.
- Waist (women): horizontally at the narrowest point between ribs and hips, at the end of a normal breath out.
- Hips (women): horizontally around the widest part of the buttocks, feet together.
- Pull the tape snug without squeezing, do not suck in, measure each site three times and use the average. Measure height without shoes.
Most strange results come from measuring the waist too high or the neck too low. If the calculator says your numbers are impossible, check those first.
How to read your result
The scale uses the body fat categories published by the American Council on Exercise (ACE):
| 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 |
An example: a man 178 cm tall with a 38 cm neck and an 86 cm waist at the navel gets about 17.3%, which is in the ACE "fitness" band. If he weighs 78 kg, that is about 13.5 kg of fat and 64.5 kg of lean mass.
Women carry more essential fat than men for hormonal and reproductive reasons, so the bands are shifted.
Mind the error range. In the original validation against underwater weighing, the standard error of estimate was about 3.5 percentage points for men and 3.7 for women (as summarised by the US Institute of Medicine in 1998). That means a result of 20% is best read as "roughly 16–24%". The outlined box on the scale shows this range so you can see when you sit near a category boundary. The method is also least accurate at the extremes: very lean, very muscular or very heavy bodies.
What you can do
- Track the trend. Measure every four weeks under the same conditions, at the same time of day. A drop of 1–2 points over a few months is real progress.
- Watch your waist. It is in the formula and is one of the best simple markers of health risk. Keeping it under half your height is a good target.
- Keep or build muscle. Strength training on 2 or more days a week, alongside 150 minutes of moderate activity, is what WHO recommends for adults. Combine it with simple checks like the balance test, or track your resting pulse with the heart rate monitor online.
Limits and when to see a doctor
This is an estimate for adults, not a measurement of fat itself and not a diagnosis. Lab methods such as DEXA or underwater weighing are more precise, and even they disagree with each other by a few points. See a doctor if your weight or waist changes quickly without a clear reason, if your result is in the obese range and you would like help, or if a very low result comes with missed periods, dizziness or feeling cold all the time.
More body checks are on our fitness tests page.
Sources
- Hodgdon, J. A., & Beckett, M. B. (1984). Prediction of percent body fat for U.S. Navy men from body circumferences and height (Report No. 84-11). San Diego, CA: Naval Health Research Center.
- Hodgdon, J. A., & Beckett, M. B. (1984). Prediction of percent body fat for U.S. Navy women from body circumferences and height (Report No. 84-29). San Diego, CA: Naval Health Research Center.
- Institute of Medicine (1998). Assessing Readiness in Military Women: The Relationship of Body Composition, Nutrition, and Health. Washington, DC: National Academy Press (Table 2-1).
- American Council on Exercise. Percent body fat norms for men and women (ACE body fat chart).
- World Health Organization (2020). WHO guidelines on physical activity and sedentary behaviour. Geneva: WHO.
Frequently asked questions
How accurate is the Navy body fat calculator?
In the original studies the standard error was about 3.5 percentage points for men and 3.7 for women compared with underwater weighing. Treat your result as a range of roughly plus or minus 3 to 4 points, shown as the outlined box on the scale.
Where do I measure my waist for the Navy method?
Men measure horizontally at the level of the navel. Women measure at the narrowest point between the ribs and hips. In both cases, measure at the end of a normal breath out without sucking in.
Why does the formula use my neck?
Neck size mostly reflects muscle and frame rather than fat. Subtracting it from the waist (and hips for women) helps separate fat from overall body size.
What is a healthy body fat percentage?
On the ACE chart, the fitness range is 14–17% for men and 21–24% for women, and the average range is 18–24% for men and 25–31% for women. Obese starts at 25% for men and 32% for women.
Is body fat percentage better than BMI?
It tells you something BMI cannot: how much of your weight is fat. But a tape estimate has its own error. Using both, plus your waist-to-height ratio, gives the fullest picture.
Can teenagers use this calculator?
The Navy equations were developed on adult service members, so results for teenagers and children are not reliable. This calculator is meant for adults.
Why is my result so low or impossible?
Usually because the waist was measured too high or the neck too low, or the units were mixed up. Re-measure each site three times and check that you chose cm or inches correctly.