How the Navy method works
The U.S. Navy needed a way to check body composition for thousands of sailors without lab equipment. Researchers at the Naval Health Research Center compared tape measurements with underwater weighing and found that a few circumferences, combined with height, predict body density well. The resulting equations are still used by the U.S. military for body-composition standards.
The idea is simple: the waist (and, for women, hips) grows with fat, while the neck mostly reflects frame and muscle. The bigger the waist relative to the neck for a given height, the higher the estimated body fat.
The formulas (centimeters)
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
The “495 / density − 450” part is the Siri equation, which turns estimated body density into a fat percentage. Inch measurements are converted to centimeters first (× 2.54).
Worked example
A man 5 ft 10 in (177.8 cm) with a 15 in neck (38.1 cm) and 34 in waist (86.4 cm):
waist − neck = 48.3 cm → log10 = 1.6836; log10(177.8) = 2.2499. Density term = 1.0324 − 0.19077 × 1.6837 + 0.15456 × 2.2499 ≈ 1.0590. BF% = 495 / 1.0590 − 450 ≈ 17.4% — the “fitness” range. At 175 lb, that's about 30.5 lb of fat and 144.5 lb of lean mass.
A woman 5 ft 5 in with a 13 in neck, 29 in waist and 39 in hips comes out at about 28.3% — “average”.
American Council on Exercise categories
| 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%+ | 25%+ |
Limitations
- An estimate from a population equation; individual error of several percentage points is normal.
- Measurement technique matters more than anything else — measure three times and use the average.
- Not validated for children, pregnant women, or people with significant swelling or amputations.
Sources: Hodgdon, J. A. & Beckett, M. B. (1984), Prediction of percent body fat for U.S. Navy men and women from body circumferences and height, Naval Health Research Center Reports 84-11 and 84-29; Siri, W. E. (1961); American Council on Exercise, body-fat categories.
Frequently asked questions
How accurate is the U.S. Navy method?
In validation studies it typically lands within about 3–4 percentage points of laboratory methods such as hydrostatic weighing or DEXA for most adults. It is less accurate for very lean, very muscular or very heavy people, and small measuring errors — especially at the waist — move the result noticeably.
Where exactly do I measure?
Neck: just below the larynx (Adam's apple), tape sloping slightly downward to the front. Waist: men at the navel; women at the narrowest point of the abdomen. Hips (women): at the widest point of the buttocks. Stand relaxed, don't pull in your stomach, keep the tape snug but not compressing the skin, and measure to the nearest half inch.
What is a healthy body fat percentage?
The American Council on Exercise lists 14–24% as fitness-to-average for men and 21–31% for women. Essential fat — the minimum needed for normal body function — is about 2–5% for men and 10–13% for women. Ranges shift with age.
Why do women need a hip measurement?
Women typically store more fat around the hips and thighs, so the Navy equations for women add hip circumference to waist and subtract neck. The men's equation uses waist minus neck only.
Is this better than BMI?
It answers a different question. BMI relates weight to height; the Navy method estimates how much of your weight is fat, so it can tell a muscular person from someone with excess fat at the same BMI. Using both, plus waist size, gives a fuller picture.
This calculator is for general information and screening only and is not medical advice. Talk to a doctor or registered dietitian before making changes based on the result, especially if you are pregnant, under 18, or managing a health condition.