Body Fat Calculator

Background.

This body fat calculator estimates the percentage of your total body mass that is adipose tissue using the U.S. Navy circumference method, a tape-measure equation developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 and adopted by the U.S. Department of Defense for fitness screening across all four service branches. Enter your sex, height, neck circumference, and waist circumference in inches — women also enter a hip measurement — and the calculator returns a single percentage plus an ACE category label ranging from Essential fat through Athletes, Fitness, Average, and Obese. The result is rounded to one decimal place and updates instantly; nothing is stored, transmitted, or required to leave your device.

The Navy method exists because the gold-standard ways of measuring body composition — dual-energy X-ray absorptiometry (DEXA), hydrostatic underwater weighing, and air-displacement plethysmography (Bod Pod) — all require specialised equipment that most people will never have access to. Skinfold callipers, the next step down in accuracy, require a trained technician and good site-to-site reproducibility; consumer bioelectrical impedance scales are convenient but their readings drift with hydration, recent food intake, skin temperature, and time of day. The circumference method occupies the practical middle ground: anyone with a soft cloth tape measure and a mirror can produce a number, and the equation was validated in 1984 against hydrostatic weighing in a Navy sample of 602 men and 214 women with mean absolute errors of roughly 3 to 4 percentage points across the range. That error is large enough that you should not chase month-to-month tenths of a percent, but small enough to be genuinely useful for tracking direction over six- to twelve-month spans of training or weight change.

The math is a pair of base-10 logarithmic regressions. For men the formula is %BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76, where waist is measured at the navel and the result is read straight off the equation. For women the formula adds the hip measurement and slightly different coefficients: %BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387, with waist measured at the narrowest point of the natural waist and hips at the widest point of the buttocks. The hip measurement is included for women because adult women carry a substantially larger and more variable share of body fat on the hips and thighs than men do; omitting it would systematically misclassify women with gynoid fat distribution. Both equations assume the measurements are taken in inches and the body fat percentage is reported as a percentage (not a fraction). Quanta's implementation evaluates the equations to full floating-point precision and then rounds the final percentage to one decimal place, which keeps the displayed result stable across browsers and avoids the cascading rounding errors that show up when intermediate values are rounded mid-calculation.

The category label uses the sex-specific ranges published by the American Council on Exercise (ACE) and widely echoed in the ACSM Position Stand on body composition. For men: Essential fat 2–5%, Athletes 6–13%, Fitness 14–17%, Average 18–24%, Obese 25% and above. For women: Essential fat 10–13%, Athletes 14–20%, Fitness 21–24%, Average 25–31%, Obese 32% and above. The female ranges sit roughly 8 to 10 percentage points higher than the male ranges at every band because women carry a biologically necessary higher minimum of sex-specific essential fat — primarily in breast tissue and around the reproductive organs — and because the typical healthy distribution of fat on the female body differs from the male pattern even at low total body fat percentages.

We want to be direct about what this calculator can and cannot tell you. It is an estimate, not a measurement. Compared with DEXA, the Navy method produces individual errors typically in the range of plus or minus 3 to 4 percentage points and occasionally larger, with the largest errors observed in very lean athletes (whose abdominal circumference is small relative to their muscular neck) and in people with central adiposity (whose waist measurement overstates body fat relative to peripheral distribution). The method also assumes a normative pattern of fat distribution; if you carry an unusual amount of fat above the waist (men) or have particularly slim or particularly wide hips for your overall composition (women), your true body-fat percentage may differ meaningfully from the calculator's estimate. The method does not work for children, where age- and sex-specific percentile charts are used instead, and it has not been validated in pregnancy, where total body water and fat distribution change too dramatically for any tape-measure equation to track reliably.

None of this means the number is useless. Used consistently — same time of day, same tape, same posture, same person doing the measuring — the Navy method is one of the cheapest reliable ways to track body composition change over months and years. A reading of 22% in March that drops to 18% by September while your scale weight stays flat is exactly the kind of recomposition signal that BMI alone cannot give you. Treat the number as a directional indicator, not a verdict, and remember that body fat percentage is one variable in a larger picture that also includes waist circumference, resting blood pressure, fasting glucose or HbA1c, lipid panel, cardiorespiratory fitness, and how you actually feel and perform day to day. The calculator below is fast, exact to the floating-point evaluation of the published equations, and uses the same coefficients that the U.S. Navy and the U.S. Marine Corps use today for body composition assessment.

What is body fat calculator?

The U.S. Navy body fat calculator is a tape-measure estimate of body fat percentage based on a pair of logarithmic regression equations developed by Lieutenant Commander James Hodgdon and Marcie Beckett at the Naval Health Research Center in San Diego in 1984 and 1985. The equations were derived by regressing body density measured via hydrostatic underwater weighing — at the time the practical gold standard for body composition — against simple circumference measurements taken with a soft cloth tape, on a sample of active-duty Navy personnel. Two separate equations were fitted, one for men using neck and waist (at the navel) plus height, and one for women using neck, natural-waist, hips, plus height. The hip measurement was added for women because the female fat-distribution pattern places a larger and more variable share of total body fat on the hips and thighs than the male pattern. The original Navy reports showed mean absolute errors of approximately 3 to 4 percentage points compared with hydrostatic weighing, which made the method accurate enough for population-level fitness screening despite its simplicity. The Department of Defense formally adopted the method in DoD Instruction 1308.3 and it remains in use across the Navy, Marine Corps, Army, and Air Force as the standard body composition assessment for personnel who exceed their service's screening weight-for-height table. Outside the military, the same equations are used by the American Council on Exercise's reference materials, by countless commercial fitness apps, and by clinicians who want a quick, equipment-free estimate to track alongside scale weight and waist circumference. The body fat percentage returned by the Navy method should be read as an estimate — not as a substitute for DEXA, hydrostatic weighing, or air-displacement plethysmography, all of which give substantially smaller individual errors but require specialised equipment and trained operators.

How to use this calculator.

1. 01Select your sex. The male equation uses neck and waist; the female equation also requires a hip measurement because adult women carry a larger and more variable share of body fat on the hips and thighs.
2. 02Enter your height in inches. A person who is 5 feet 10 inches tall is 70 inches; 6 feet 0 inches is 72 inches.
3. 03Measure your neck circumference just below the larynx (Adam's apple) with the tape sloping slightly downward to the front. Do not flex the neck. Record to the nearest quarter inch.
4. 04Measure your waist. Men: place the tape horizontally at the level of the navel. Women: use the narrowest point of the natural waist, typically about an inch above the navel. Breathe normally and do not suck in.
5. 05Women only: measure hip circumference at the widest point of the buttocks, with feet together and weight evenly distributed. The tape must be horizontal — check in a mirror or have someone help.
6. 06Read the body fat percentage in the results panel, rounded to one decimal place, along with the ACE category label (Essential fat, Athletes, Fitness, Average, or Obese).
7. 07For consistent tracking over time, take measurements first thing in the morning before eating or drinking, use the same tape, and measure each site at least twice — accept the reading only if two consecutive measurements agree within a quarter inch.

The formula.

The U.S. Navy method uses two sex-specific logarithmic regression equations originally published by Hodgdon and Beckett (1984) at the Naval Health Research Center. For men the equation is %BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76, where all three circumferences are in inches and log10 is the common (base-10) logarithm. For women a hip measurement is added and the coefficients are different: %BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387. Both equations return body fat as a percentage of total body mass (e.g. 18.4, not 0.184). The equations were derived by regressing body density measured via hydrostatic underwater weighing against the circumference and height inputs on a Navy sample of 602 men and 214 women, then converting body density to percent fat using the Siri equation. The model fit reported in the original technical reports had mean absolute errors of roughly 3.5 percentage points for men and 3.7 for women across the full sampled range. This calculator evaluates the equations at full floating-point precision and rounds only the final displayed percentage to one decimal place. The ACE category mapping uses the sex-specific bands long published by the American Council on Exercise and widely echoed in ACSM educational materials. For men the bands are: Essential fat 2–5% (code 0), Athletes 6–13% (code 1), Fitness 14–17% (code 2), Average 18–24% (code 3), Obese 25% and above (code 4). For women: Essential fat 10–13% (code 0), Athletes 14–20% (code 1), Fitness 21–24% (code 2), Average 25–31% (code 3), Obese 32% and above (code 4). Boundaries are inclusive at the lower end and exclusive at the upper end except for the open-ended Obese band; a male reading of exactly 14.0% lands in Fitness, and a female reading of exactly 25.0% lands in Average.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Hodgdon JA, Beckett MB — Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center Report No. 84-11 (1984). The primary technical report deriving the male circumference equation used by this calculator.
2. [2]Hodgdon JA, Beckett MB — Prediction of percent body fat for U.S. Navy women from body circumferences and height. Naval Health Research Center Report No. 84-29 (1984). The companion technical report deriving the female circumference equation.
3. [3]American College of Sports Medicine — ACSM's Guidelines for Exercise Testing and Prescription (11th edition, 2021), Chapter on body composition assessment. Discusses circumference methods, skinfold equations, and DEXA reference standards, and reports typical standard errors for each.
4. [4]American Council on Exercise (ACE) — Percent body fat norms for men and women, the sex-specific Essential / Athletes / Fitness / Average / Obese bands used by this calculator's category mapping.
5. [5]Jackson AS, Pollock ML — Practical assessment of body composition. The Physician and Sportsmedicine 13(5):76–90 (1985). Foundational peer-reviewed comparison of skinfold, circumference, and hydrostatic methods that informed the validation framework used for the Navy equations.
6. [6]Friedl KE — Body composition and military performance — many things to many people. Journal of Strength and Conditioning Research 26 Suppl 2:S87–100 (2012). Peer-reviewed review of the Navy and Army body composition standards, their derivation, and their accuracy against DEXA in service member populations.
7. [7]National Institutes of Health — Assessing Your Weight and Health Risk (NHLBI). Patient-facing reference that pairs BMI screening with waist circumference and other body composition indicators in cardiometabolic risk assessment.
8. [8]American Academy of Family Physicians (AAFP) — Diagnosis and Management of Obesity in Adults. Clinical reference covering body composition assessment in primary care, including the role of circumference-based estimates alongside BMI.