Important: Not medical advice. This report is a general estimate from a published formula. It is not medical advice, a diagnosis, or a treatment plan, and it doesn't account for your health history, medications, pregnancy or other conditions. Talk to a doctor, midwife or registered dietitian before making health decisions based on it. Use of this report is at your own risk; see aiocalc.com/disclaimer and aiocalc.com/terms.

Health

Body Fat Calculator

Estimate your body fat percentage from tape-measure measurements, using the US Navy method. All you need is a flexible tape and a minute in front of a mirror.

Updated October 11, 2026 · 4 cited sources

Your measurements

Estimated body fat · US Navy method

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Category: —

Enter your measurements to see your estimate.

Likely range

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± the Navy equations' standard error

ACE range for your category

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Where you fall on the ACE scale

Show the math

Body fat % = 495 ÷ body density − 450 (Siri), density from the Navy equation in cm

Estimate only, not medical advice — the tape-measure method is less precise than lab methods like DEXA, especially for atypical body proportions.

How the Navy method works

The US Navy circumference method estimates body density from a log-linear formula using neck, waist (and for women, hip) measurements relative to height, then converts that density to a body fat percentage. It's the method the US military uses for fitness assessments precisely because it requires no special equipment — just a tape measure.

The equations come from Hodgdon and Beckett at the Naval Health Research Center (1984), who measured sailors' circumferences and compared them with body density from underwater weighing. In centimetres, the men's equation is density = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height); the women's is 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.221 × log10(height). Siri's equation, 495 ÷ density − 450, then turns density into a percentage. Open "Show the math" under the result to see your own numbers plugged in.

How to measure neck, waist & hip

Use a flexible, non-stretch tape, stand relaxed with your weight on both feet, and keep the tape level unless noted. US military tape tests use these landmarks:

Neck: just below the larynx (Adam's apple), with the tape at right angles to the neck so it slopes slightly down toward the front. Look straight ahead and don't flare your neck or shrug.
Waist, men: around the abdomen at the level of the navel, at the end of a normal, relaxed breath out. Don't suck in.
Waist, women: at the narrowest point of the abdomen, usually a little above the navel, also at the end of a relaxed breath out.
Hip (women only): around the widest part of the buttocks, with feet together and the tape parallel to the floor.

Keep the tape snug against the skin without compressing it, and measure at a consistent time of day (measurements can shift slightly with hydration and food intake) for the most reliable tracking over time. Measure over bare skin or thin clothing, take each site two or three times, and average the readings. If your tape is in centimetres and you want inches, switch the unit toggle and the values convert, or use the cm to inches converter.

How accurate is it?

In the original Navy studies, the equations were checked against underwater weighing in groups that weren't used to build them. The standard error was about 2.7 percentage points for men and about 4 points for women, so a reading of 22% is best read as "roughly 19–25%" for a man. That's accurate enough to track meaningful trends over weeks and months, though not a clinical-grade measurement. It tends to be less reliable at the very lean or very heavy ends of the range, or for unusual body proportions.

Later military research found the tape method useful for sorting people into categories: the Department of Defense's Human Performance Resources by CHAMP (HPRC) site reports a Marine Corps study in which it classified about 92% of Marines the same way as a DEXA scan. Because every tape reading carries some error, the trend across several measurements tells you more than any single number.

What should my body fat percentage be?

For men: 2-5% is essential fat, 6-13% athletic, 14-17% fitness, 18-24% average, 25%+ obese. For women: 10-13% essential fat, 14-20% athletic, 21-24% fitness, 25-31% average, 32%+ obese — women naturally carry more essential fat than men for hormonal and reproductive reasons, which is why the healthy ranges differ.

American Council on Exercise body fat categories for men and women
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 up	32% and up

These ACE categories are the same for every adult age. Many adults gain body fat as they age, so an older adult in the "average" band isn't unusual. Going below the essential-fat range isn't a goal; it can disrupt hormones and, in women, menstrual cycles.

Body fat % vs. BMI

BMI uses only height and weight, so it can't tell muscle from fat: a muscular person can land in the "overweight" BMI band with a low body fat percentage, while someone with little muscle can have a normal BMI and a high body fat percentage. A tape-measure estimate adds information about where weight is carried, especially around the waist. Neither replaces a clinical assessment, but used together they give a fuller picture than either alone.

Using your result

Your body fat percentage also tells you your lean mass: weight × (1 − body fat ÷ 100). At 180 lb and 20% body fat, that's 144 lb of lean mass. Lean mass feeds the Katch-McArdle equation mentioned on the BMR calculator, and it's a sensible base for a protein target if you carry a lot of fat; the protein calculator accepts any weight you enter. To change body composition, pair a modest calorie target from the calorie calculator with strength training.

Sources

The formulas on this page come from the published research below. Last updated October 11, 2026. This calculator gives estimates for general information. It isn't medical advice, so check anything important with a qualified professional.

Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men (Report No. 84-11) and women (Report No. 84-29) from body circumferences and height. San Diego, CA: Naval Health Research Center; 1984.Used for: US Navy circumference equations for body density
Siri WE. Body composition from fluid spaces and density: analysis of methods. In: Brozek J, Henschel A, eds. Techniques for Measuring Body Composition. Washington, DC: National Academy of Sciences; 1961:223–244.Used for: Converting body density to body fat percentage
American Council on Exercise (ACE). Body fat percentage norms for men and women.Used for: Essential / athletic / fitness / average / obese categories
Human Performance Resources by CHAMP (HPRC), Uniformed Services University. How the military measures body composition.Used for: Tape-measurement method and how it compares with DEXA

Frequently asked questions

Why does my body fat result jump around from week to week?

A half-inch change in waist measurement can move the estimate by a point or more, and the waist itself shifts with meals, bloating and how hard you pull the tape. Measure first thing in the morning before eating, take each measurement two or three times and use the average, then compare monthly trends rather than single readings.

Why don't men need a hip measurement?

The Navy researchers fitted separate equations for each sex, keeping whichever measurements best predicted body density in their study groups. For men, the abdomen minus the neck was enough. For women, adding the hip improved the prediction, because women tend to store more fat around the hips and thighs. Using the female equation without a hip value would be meaningless, so the calculator asks for one.

Can I use this body fat calculator if I'm pregnant or under 18?

It isn't a good fit for either. The Navy equations were built from measurements of adult service members, so they don't account for pregnancy, when the waist grows for reasons that have nothing to do with body fat, or for children and teens, whose body proportions are still changing. A doctor can use growth charts or other methods instead.