Body Fat Calculator
Estimate body fat percentage by circumference or BMI
Estimate your body fat percentage two ways: the US Navy circumference method (neck, waist, hip) and a BMI-based (Deurenberg) estimate. A reference tool for gauging body fat with just a tape measure or your weight — no body fat scale or InBody needed. 100% in your browser.
Result
Enter your measurements to see the result.
- Category
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How the two methods differ
- The US Navy method uses circumference measurements (neck and waist, plus hip for women) and is widely used by the military and fitness industry because it's fairly reliable without needing a body fat scale.
- The BMI-based (Deurenberg) estimate only needs height, weight, age, and sex, making it simpler, but it can be less accurate for people with unusually high or low muscle mass.
- Both methods have a wider margin of error than direct body composition measurement (DEXA, hydrostatic weighing, etc.) — roughly ±3–4 percentage points — and can differ from your actual value depending on body shape and muscle mass.
Caveats
- This is a reference estimate, not a medical diagnosis. For an accurate measurement, use dedicated equipment like an InBody scan or DEXA scan.
- For consistent circumference readings, measure at the same time of day (e.g. right after waking up) using the same technique each time.
- This calculator is intended for adults (18+). Don't use it for children or teens who are still growing.
- Your numbers are never uploaded — everything is calculated inside this browser.
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About Body Fat Calculator
Body fat percentage is the share of your weight that is fat. Measuring it properly takes a DEXA scan, underwater weighing or an air-displacement pod. This calculator estimates it from things you can measure at home, using two published equations: the US Navy circumference method (neck, waist, hips for women, and height) and the Deurenberg equation (BMI, age and sex).
The two equations take different inputs, so they fail in different ways. Running both on the same person is useful: when they disagree by a lot, the disagreement usually tells you something about body shape.
Where the Navy tape method came from
The equations were published in 1984 by J. A. Hodgdon and M. B. Beckett at the Naval Health Research Center in San Diego, in technical reports for men and for women. The Navy needed to check body composition for a very large number of sailors without lab equipment, so the equations use only sites a tape measure can reach.
The metric men's equation is 495 / (1.0324 − 0.19077·log10(waist − neck) + 0.15456·log10(height)) − 450. The outer part, 495/density − 450, is the Siri equation, which turns whole-body density into percent fat. The bracket is an estimate of body density from circumferences. The waist grows as fat accumulates, while neck size mostly reflects frame and muscle, so waist minus neck is a rough proxy for the fat-driven part of your girth. Women store more fat on the hips and thighs, so their version uses waist + hip − neck.
Because of the logarithm, the equation breaks when the waist is not larger than the neck. That is why the calculator returns no result in that case rather than a nonsense number.
How much one centimetre matters
A man 178 cm tall with a 38 cm neck and 86 cm waist comes out at 17.2%. Measure the waist at 88 cm and the result is 18.7%; at 84 cm it is 15.7%. Two centimetres of tape moves the answer by about 1.5 percentage points. Reading the neck 1 cm small (37 cm) raises it to 18.0%.
Those differences are easy to produce: tape slightly above or below the navel, a breath held in, a big lunch. If you are tracking progress, measure under the same conditions each time (morning, before eating, relaxed exhale, same landmarks) and look at the trend across several weeks. A drop from 17% to 16% in one week is within measurement noise.
The Deurenberg equation: why age is in it
Paul Deurenberg and colleagues at Wageningen Agricultural University measured body fat by densitometry in 1,229 people aged 7 to 83 and related it to BMI. Their adult equation, published in the British Journal of Nutrition in 1991, is: body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex = 1 for men, 0 for women). The standard error of estimate was 4.1 percentage points.
The age term is there because people lose muscle and gain fat as they get older while BMI can stay the same. A man with BMI 24 comes out at 19.5% at age 30 and 26.4% at age 60. The 10.8 sex term says that at the same BMI, women carry roughly 11 points more body fat.
The authors noted that the equation slightly overestimated body fat in obese subjects, and that children 15 and under need a different formula. This calculator applies the adult version only.
When the two methods disagree
A gap of more than about five points between the two results usually points to one of these.
- Muscular builds: a high BMI pushes the Deurenberg estimate up, while a trim waist keeps the Navy estimate low. The Navy number is usually closer here.
- Normal weight with a belly: BMI looks fine, so Deurenberg is low; the waist is large, so the Navy estimate is high. The Navy number flags what BMI misses.
- Thick necks: lifters and wrestlers with developed traps and neck muscles can get Navy results that are too low, since neck size is subtracted.
- Pear-shaped women: large hip measurements raise the women's Navy estimate substantially.
Reading the categories
The bands on the result bar (essential fat, athletes, fitness, average, obese) follow the chart popularised by the American Council on Exercise. ACE puts the typical non-athlete at 18–24% for men and 25–31% for women. Women's essential fat is much higher because some fat is tied to reproductive function. The bands are not age-adjusted and are not a medical diagnosis.
Worked examples
Man: tape method versus BMI method
- Input
Male, 35, 178 cm (5 ft 10 in), 76 kg (168 lb), neck 38 cm, waist 86 cm- Output
Navy 17.2% (fitness) · BMI 24.0 → Deurenberg 20.6% (average)
Deurenberg: 1.2 × 23.99 + 0.23 × 35 − 10.8 − 5.4 = 20.6. The 3.4-point gap is within the equation's 4.1-point standard error. His waist is small relative to his height, which pulls the Navy estimate lower.
Woman: effect of the hip measurement
- Input
Female, 165 cm, neck 33 cm, waist 75 cm, hips 95 cm- Output
Navy 26.9% (average)
Waist + hip − neck = 137 cm. Reading the hips 2 cm larger (97 cm) gives 27.9%, about one point higher. Waist and hips carry the same weight in the women's formula, so an error at either site has the same effect.
How to use it
- Choose sex and units (cm or inches).
- Enter height, neck and waist; women also enter hips.
- Add weight and age to get the BMI-based estimate alongside.
Frequently asked questions
Where exactly should I measure my waist?
The usual convention is the navel for men and the narrowest point of the waist for women, the neck just below the Adam's apple, and the hips at the widest point of the buttocks, with the tape level. Using the same landmark every time matters more than which one you pick.
My smart scale says something different. Which is right?
Both are estimates. Bioimpedance scales shift with hydration; the tape method shifts with measuring technique. Without a reference method such as DEXA, pick one approach and follow its trend.
Is the Navy method accurate for athletes?
It tends to be better than BMI-based estimates for muscular people, but a very thick neck can push results too low. For competitive athletes, a lab method is worth it.
Can teenagers use it?
Deurenberg's paper gave a separate equation for children 15 and under. The formulas here are the adult versions, so treat teenage results as rough.
Related guides
Sources
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center Report 84-11, 1984 (DTIC)
- Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr 1991;65:105-114
- American Council on Exercise, Body fat percentage averages for men and women
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About the results
Results depend on your inputs and assumptions. Check official sources or a qualified professional before using them for medical decisions, contracts or filings.