Body Fat Calculator (US Navy method)

Understanding the US Navy body fat calculator — concept and principle

The US Navy method estimates body fat percentage from a few tape-measure circumferences and your height. It was built to assess service members without laboratory equipment. This calculator gives your body fat percentage, splits your weight into fat mass and lean mass, and places the result in the American Council on Exercise (ACE) categories.

Waist minus neck and height go into the Navy formula: 17.71% of 75 kg is about 13.3 kg of fat.

How it is calculated

All measurements are in centimetres.

Men: %BF = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
Women: %BF = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450
Fat mass = weight × %BF ÷ 100; lean mass = weight − fat mass.
ACE categories, men: essential under 6%, athletes 6–13%, fitness 14–17%, average 18–24%, obese 25% and above. Women: essential under 14%, athletes 14–20%, fitness 21–24%, average 25–31%, obese 32% and above.

The principle

Hodgdon and Beckett measured body density in Navy personnel by underwater weighing, the reference method of the time, and found which tape measurements predicted it best. For men, waist minus neck works well: the waist grows with abdominal fat, while the neck reflects frame and muscle. Women carry more fat on the hips, so hip circumference is added. The denominator of the formula is the predicted body density, and 495 ÷ density − 450 is the Siri equation, which converts density into a fat percentage by assuming fixed densities for fat and lean tissue.

Worked example

Inputs: male, height 175 cm, neck 38 cm, waist 86 cm, weight 75 kg.
Waist − neck = 48; log₁₀(48) = 1.6812 and log₁₀(175) = 2.2430.
Density = 1.0324 − 0.19077 × 1.6812 + 0.15456 × 2.2430 = 1.05835.
%BF = 495 ÷ 1.05835 − 450 = 17.71%.
Fat mass = 75 × 0.1771 = 13.28 kg; lean mass = 75 − 13.28 = 61.72 kg.
17.71% is under 18%, so the ACE category is fitness.

Each point is the calculator re-run with only “Neck (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Neck (cm)	Body fat
9.5	35.42%
19	30.25%
28.5	24.41%
38	17.71%
47.5	9.77%
57	-0.04%
76	-33.44%

How to read the result

Where does your result fall?

Expect an error of about 3 to 4 percentage points against laboratory methods, so 17.71% means roughly 14–21%. The tool is most valuable for tracking change: if your waist shrinks while your neck stays the same, the estimate falls.

Common mistakes

Measuring the neck above the Adam's apple instead of just below the larynx, or the waist above the navel.
Pulling the tape tight enough to compress the skin.
Entering a hip value for a man; the male formula ignores it.

Keep in mind: This is an estimate, not a diagnosis. It can be less accurate for very muscular or very lean people and for people with unusual fat distribution. Talk to a doctor about your results if you have health concerns.

Frequently asked questions

How accurate is the Navy method compared with DEXA?

Typical error is about ±3–4 percentage points, similar to good skinfold measurements.

Where should women measure the waist?

Navy instructions use the narrowest part of the natural waist for women and the navel level for men.

Why does the formula use logarithms?

Body density changes less and less as circumference keeps growing; a logarithm captures that curve.

Sources: Hodgdon JA, Beckett MB, Prediction of percent body fat for U.S. Navy men and women from body circumferences and height, Naval Health Research Center, San Diego (1984) · Siri WE, Body composition from fluid spaces and density (1961)

Checked 8 Nov 2026. Hodgdon & Beckett (1984), Naval Health Research Center.

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