Body Fat Calculator

Estimate your body fat percentage from a few tape measurements with the U.S. Navy formula, and see your fat mass, lean mass and ACE category, with the working.

Example Enter your own measurements above.

19.4% body fat

Average for men (ACE: 18–24%) · U.S. Navy tape method — an estimate, not a measurement

Fat mass

35.9 lb

19.4% of your weight

Lean mass

149.1 lb

muscle, bone, organs, water

Working

Height: 5 ft × 12 + 10 in = 70 in
waist − neck = 36 − 16 = 20 in
log10(20) = 1.30103; log10(70) = 1.84510
Body fat = 86.010 × 1.30103 − 70.041 × 1.84510 + 36.76 = 111.9 − 129.23 + 36.76 = 19.43 → 19.4%
ACE category for 19.4% (men): Average (18–24%)
Fat mass = 185 lb × 19.43% = 35.9 lb
Lean mass = 185 − 35.9 = 149.1 lb

For adults. Not for use during pregnancy. An estimate for general information, not medical advice or a diagnosis — talk to a doctor about your weight and health.

How to use the body fat calculator

Choose your sex and units (US inches and pounds are the default), then enter your height, neck and waist — and your hips if you are a woman. Weight is optional; add it to see how many pounds of fat and lean mass the percentage represents. The result, your place on the ACE scale and every step of the working update as you type. Until you enter your own numbers the example shows a man who is 5 ft 10 in with a 16 in neck and 36 in waist. Nothing you type leaves your browser.

Accurate measurements matter more than anything else here. Use a flexible tape that does not stretch:

Neck: just below the larynx (Adam’s apple), with the tape at right angles to the neck.
Waist (men): horizontally at the level of the navel, relaxed, at the end of a normal breath out.
Waist (women): horizontally at the narrowest part of the abdomen, at the end of a normal breath out.
Hips (women): around the widest part of the buttocks, keeping the tape level.
Height: standing straight, without shoes.

Measure on bare skin, snug but without pressing into the skin. Take each measurement two or three times and use the average; a half-inch error in the waist can move the result by a percentage point or more.

The U.S. Navy body fat formula

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
(all measurements in inches)

Fat mass  = weight × % body fat ÷ 100
Lean mass = weight − fat mass

James Hodgdon and Mary Beckett developed the equations at the Naval Health Research Center in San Diego in 1984, using tape measurements and underwater (hydrostatic) weighing of Navy personnel — 602 men aged 18 to 56 in the men’s study. The log forms above are the versions written into Department of Defense Instruction 1308.3 (2002). The constants only work with inches. A centimeter version exists, built on body density and the Siri equation with different constants, and it gives slightly different results; this calculator converts centimeters to inches (÷ 2.54) and uses the inch equations, so both unit systems agree. For quick conversions, see inches to centimeters.

Worked examples

A man is 5 ft 10 in (70 in) tall with a 16 in neck and 36 in waist, and weighs 185 lb. Waist − neck = 20 in. log10(20) = 1.30103 and log10(70) = 1.84510, so body fat = 86.010 × 1.30103 − 70.041 × 1.84510 + 36.76 = 19.43, or 19.4% — in the ACE “average” range for men (18–24%). Fat mass = 185 × 19.43% = 35.9 lb, and lean mass = 185 − 35.9 = 149.1 lb.

A woman is 5 ft 5 in (65 in) tall with a 13 in neck, 28 in waist and 39 in hips, and weighs 145 lb. Waist + hip − neck = 54 in, so body fat = 163.205 × 1.73239 − 97.684 × 1.81291 − 78.387 = 27.3% (ACE “average”, 25–31% for women). Fat mass = 39.5 lb; lean mass = 105.5 lb.

Body fat categories

American Council on Exercise body fat categories
ACE category	Women	Men
Essential fat	10–13%	2–5%
Athletes	14–20%	6–13%
Fitness	21–24%	14–17%
Average	25–31%	18–24%
Obese	32% or more	25% or more

These are the American Council on Exercise’s general categories for adults. Essential fat is the minimum the body needs to work normally, and women carry more of it than men, so every band is higher for women. ACE prints whole-number ranges; a result that falls between two bands, such as 17.5% for a man, is placed in the lower one. The categories describe fitness norms, not a medical diagnosis.

How accurate is it?

Original validation. In the 1984 Navy study, the men’s equation matched underwater weighing with a correlation of about 0.90 and a standard error of about 2.7 percentage points.
Against DXA scans. A 2022 study of 609 US Marines found circumference-based estimates useful for screening but not research-grade: for individuals, lean people tended to be overestimated and people with more body fat underestimated.
Tracking change. A 2023 study of military trainees found tape estimates averaged about 6 percentage points below DXA and missed much of the change in body fat over eight weeks, so the method is poor at detecting small, short-term changes.
Body shape. The formula only sees your neck, waist, hips and height. People who store fat mainly on the thighs, or who have unusually large or small necks, can be misjudged.

Other ways to measure body fat

DXA (dual-energy X-ray absorptiometry): a low-dose X-ray scan that measures fat, lean tissue and bone, region by region. One of the most accurate methods, but it costs money and needs a clinic or lab.
Underwater (hydrostatic) weighing: the reference method the Navy equations were built on; you are weighed fully submerged after breathing out.
Air displacement plethysmography: a sealed chamber (such as the Bod Pod) measures your body volume instead of dunking you.
Bioelectrical impedance (BIA): smart scales and handheld devices. Quick, but readings shift with hydration, meals and exercise.
Skinfold calipers: cheap and portable, but accuracy depends heavily on the tester’s skill.

If you only want a quick health screen, waist size relative to height is a simple option — the Navy now uses waist-to-height ratio as its first check, and the Army switched to a one-site waist tape test in 2023–2024. If you are in the military, follow your service’s current official procedure rather than this calculator. For weight-based measures, see the BMI calculator and the ideal weight calculator; to turn a goal into daily numbers, use the calorie calculator.

Frequently asked questions

How accurate is the U.S. Navy body fat method?

It is a reasonable estimate, not a measurement. In the Navy’s original validation against underwater weighing, the men’s equation had a standard error of about 2.7 percentage points. Later studies comparing tape equations with DXA scans found larger errors for some people — body fat tends to be underestimated in people with the most fat and overestimated in the leanest.

Why does the calculator need my neck measurement?

Neck size mostly reflects muscle and frame rather than fat. Subtracting it from the waist (and hips, for women) helps the equation separate fat around the middle from overall body size, so a big-framed, muscular person is not scored the same as someone with the same waist but a slimmer build.

What is a healthy body fat percentage?

There is no single official cut-off. The American Council on Exercise lists 18–24% as average for men and 25–31% for women, with 25% or more (men) and 32% or more (women) classed as obese. Your doctor will also look at waist size, blood pressure, blood tests and family history.

Does the Navy still use this formula?

Not for its own sailors. Guidance published at the end of 2025 replaced the neck-and-waist tape test with a waist-to-height ratio screen and, if needed, a table based on height, waist and weight. The Army moved to a one-site waist test in 2023–2024. The classic Navy equations are still widely used as a free home estimate.

Can I measure in centimeters?

Yes. Switch to metric and enter centimeters and kilograms. The equation’s constants are for inches, so the calculator divides each length by 2.54 first; the working shows the conversion. Published centimeter versions of the formula use different constants and give slightly different answers.

Why is my result different from my smart scale?

Smart scales use bioelectrical impedance, which is affected by how hydrated you are, and the tape method depends on where the tape sits. Both have errors of several percentage points. To track change, pick one method, measure the same way at the same time of day, and look at the trend over months.

Sources

Last reviewed 24 September 2026 · Built and checked by the Reeliy team · How we test our tools