Body Fat Percentage Calculator
Estimate your body fat percentage from a few tape-measure measurements using the U.S. Navy method — no scale or scan required. Free and instant.
Uses the U.S. Navy circumference method (Hodgdon & Beckett, 1984) — no scale or body weight is needed, only neck, waist, height, and (for women) hip.
Published accuracy studies put this method within roughly 3–4 percentage points of more precise methods (DEXA, hydrostatic weighing) for most adults — treat the result as an estimate, not an exact figure.
Categories shown are the American Council on Exercise (ACE) bands, the same ones widely used by trainers and fitness assessments.
Not validated for children, pregnancy, or people with unusually high muscle mass, where circumference-based estimates are known to be less reliable.
All measurements are converted to centimetres internally — the underlying formula is unit-specific, and this calculator applies the correct centimetre-calibrated constants rather than reusing inch-calibrated ones on cm values (or vice versa), a mix-up that would silently produce a very wrong result.
Informational only — not medical advice. This is a population-level estimate, not a diagnosis. Read the full disclaimer.
This estimates body fat from circumference measurements using the U.S. Navy method — a validated field estimate, not a lab measurement.
Measure at the same time of day, before eating, with a flexible tape held snug but not compressing the skin — measurement technique affects the result more than almost anything else.
A free, equipment-light estimate to track over months — not a diagnostic measurement, and not a substitute for DEXA, hydrostatic weighing, or a clinician.
How it works
What is the U.S. Navy body fat method?
The U.S. Navy method estimates body fat percentage from a small set of circumference measurements — neck, waist, height, and, for women, hip — without needing a scale, a skinfold caliper, or any lab equipment. It was developed by Hodgdon and Beckett at the Naval Health Research Center in 1984, originally to give the Navy a fast, low-cost way to estimate body composition for large numbers of personnel where DEXA scans or hydrostatic (underwater) weighing were impractical. Because it only needs a flexible tape measure, it remains one of the most widely used field methods for estimating body fat outside a lab, and it is what this calculator uses.
The formula
The method uses a regression equation fitted to circumference measurements and height, run through a base-10 logarithm. For men:
Body fat % = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
For women, the formula adds hip circumference and uses different constants entirely:
Body fat % = 495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
All measurements must be in the same unit for a given constant set — this calculator's constants are calibrated for centimetres, and a differently-calibrated set of constants (486.010/70.041/36.76-style coefficients) exists for inches. The two are not interchangeable: plugging inch measurements into the centimetre-calibrated formula, or vice versa, produces a plausible-looking but badly wrong number rather than an obvious error. This calculator always converts your input to centimetres first, regardless of which units you enter, so this mix-up can't happen.
Why waist minus neck?
The formula is really estimating body density from the *difference* between waist and neck circumference, scaled by height. A larger neck relative to waist — or a smaller gap between the two — statistically correlates with lower body fat in the population the formula was derived from, and a wider waist relative to neck correlates with higher body fat. It is an empirical regression fit to real measured body-fat data (originally validated against hydrostatic weighing), not a first-principles physical model — which is exactly why it is described as an *estimate*, with a known margin of error, rather than a direct measurement.
Why does the formula need hip circumference for women, but not men?
Body fat tends to distribute differently between men and women — women on average carry proportionally more fat around the hips and thighs, while men on average carry more around the abdomen. The original Navy study found that adding hip circumference meaningfully improved the formula's accuracy for women specifically, while it added no meaningful accuracy for men, so the two sexes ended up with genuinely different equations rather than one equation with a sex-specific adjustment factor bolted on.
How accurate is it?
Validation studies comparing the Navy method against hydrostatic weighing and DEXA scans have generally found it accurate to within about 3 to 4 percentage points for most adults of typical build — good enough to be useful for tracking change over time, but not precise enough to be treated as a lab-grade number. Its accuracy tends to degrade at the extremes: very lean, very heavy, or unusually muscular individuals (where a large waist-to-neck ratio comes from muscle rather than fat, or vice versa) are the cases where this method and a DEXA scan are most likely to disagree.
What are the ACE body fat categories?
The American Council on Exercise (ACE) publishes the most commonly cited category chart, and it differs meaningfully by sex — a direct result of women's higher essential fat requirement:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Acceptable | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
"Essential fat" is the minimum fat the body needs for normal hormonal and organ function — going meaningfully below it, for either sex, carries real health risk rather than being an athletic ideal to chase.
Why is essential fat so much higher for women?
Essential fat includes fat tied to reproductive and hormonal function — most notably around the breasts, uterus, and other reproductive tissue — that has no male equivalent. This is a genuine physiological difference, not a difference in fitness standards, which is why the categories use different numeric ranges for each sex rather than one shared scale.
How do I measure neck, waist, and hip correctly?
Measurement technique affects the result more than almost any other factor, since a centimetre of slack or over-tightening on a tape measure shifts the waist-minus-neck gap the whole formula is built on:
- Neck: measure just below the larynx (Adam's apple), with the tape sloping very slightly downward at the front, as most guidance for this method specifies.
- Waist: measure at the narrowest point of the torso, or at navel level if there is no clear narrowing — pick one landmark and use it consistently if you re-measure later.
- Hip (women): measure at the widest point around the hips and buttocks.
- General technique: use a flexible (cloth or vinyl) tape, keep it parallel to the floor, snug against the skin but not compressing it, and measure at the end of a normal exhale rather than while holding your breath.
How should I use this estimate?
As with any single-number health metric, the most useful signal is usually the *trend*, not one isolated reading. Re-measuring under the same conditions — similar time of day, similar hydration, the same landmarks — every few weeks will show a meaningful change in body composition far more reliably than comparing two individual readings taken under different conditions. Pairing it with how your clothes fit, energy levels, or performance in physical activity gives a fuller picture than any one number, circumference-based or otherwise.
This calculator runs entirely in your browser. Your measurements are never transmitted or stored.
FAQ
How accurate is the U.S. Navy body fat method?
Validation studies comparing it against hydrostatic weighing and DEXA scans generally find it accurate to within about 3–4 percentage points for most adults of typical build. It is a useful estimate for tracking change over time, not a lab-grade measurement.
Why do I need a hip measurement if I select "female" but not "male"?
The original 1984 Navy study found that adding hip circumference meaningfully improved accuracy for women specifically, reflecting how body fat tends to distribute differently by sex, while it added no meaningful accuracy for men — so the two sexes ended up with genuinely different formulas.
Can I mix centimetres and inches in the same calculation?
No — and this calculator prevents that mistake for you. The formula's constants are calibrated for one unit system; using inch measurements with centimetre-calibrated constants (or the reverse) produces a plausible-looking but badly wrong result. Whichever units you enter, this tool converts everything to centimetres internally before running the one centimetre-calibrated formula.
Where exactly do I measure my neck, waist, and hip?
Neck just below the larynx (Adam's apple); waist at the narrowest point of the torso, or navel level if there is no clear narrowing; hip (for women) at the widest point around the hips and buttocks. Use a flexible tape, snug but not compressing the skin.
Why does my result look different from another body fat calculator online?
Small differences in exactly where and how tightly you measure will shift the result more than most people expect, since the formula is built on the small gap between waist and neck. Different calculators can also silently mix up the inch- and centimetre-calibrated constants — check that any calculator you compare against states which unit its formula expects.
What are the ACE body fat categories?
For men: Essential fat 2–5%, Athletes 6–13%, Fitness 14–17%, Acceptable 18–24%, Obese 25%+. For women: Essential fat 10–13%, Athletes 14–20%, Fitness 21–24%, Acceptable 25–31%, Obese 32%+. Women's bands run higher throughout because women require more essential fat for hormonal and reproductive function.
Is this as accurate as a DEXA scan or hydrostatic weighing?
No — those are direct or near-direct measurements, while this is a circumference-based estimate with a known ~3–4 percentage point margin of error. It is meant as a free, repeatable field estimate, not a replacement for a clinical body-composition scan.
Does this method work well for very muscular people?
Less reliably. The formula assumes a typical relationship between waist-to-neck ratio and body fat; someone with an unusually muscular neck or waist can produce a result that disagrees noticeably with a DEXA scan, since the formula can't distinguish muscle from fat any more directly than BMI can.
How often should I re-measure?
Every few weeks, under similar conditions (similar time of day, hydration, and measurement landmarks), is enough to see a genuine trend. A single reading is far less informative than the direction it moves over time.
Is my data sent anywhere?
No. The entire calculation happens in your browser. Nothing you enter is transmitted, logged, or stored.
Why is essential fat so much higher for women than men?
Essential fat includes fat tied to reproductive and hormonal function — most notably around the breasts, uterus, and other reproductive tissue — that has no male equivalent. It's a physiological difference, not a difference in fitness standards.