How it works
U.S. Navy (men): %fat = 495 ÷ (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450
The Navy method fits body-fat percentage to the logarithm of body circumferences, all measured in inches. For men it uses waist minus neck and height: 495 ÷ (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450. A man with a 34-inch waist, 16-inch neck and 68-inch height comes out at about 10%. For women, hips are added and the constants change: 495 ÷ (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.22100·log₁₀(height)) − 450. We convert centimetres to inches internally, so the result is the same whichever units you use. Because the model relies only on circumferences, accurate tape placement matters: measure the neck just below the larynx, the waist at the navel (men) or narrowest point (women), and the hips at their widest. The percentage is then placed into the American Council on Exercise (ACE) bands — essential fat, athletes, fitness, average and above average — which differ for men and women.
Sources
- U.S. Navy circumference method 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, Report No. 84-11, 1984.
- ACE body-fat categories American Council on Exercise (ACE) body-fat norms — men: essential 2–5%, athletes 6–13%, fitness 14–17%, average 18–24%, above 25%+; women shifted ~8–10 points higher.
- Circumference vs. reference methods Tape-based estimates approximate hydrostatic weighing and DEXA but carry several points of error; they are best for tracking change over time, not single-reading precision.
FAQ
How does the Navy body-fat method work?
It estimates body fat from the logarithm of a few circumferences and your height. Men use neck, waist and height; women add the hips. It was developed by the U.S. Navy as a quick screen requiring only a tape measure, and it tracks more expensive methods reasonably well for most people.
How accurate is a tape-measure body-fat estimate?
It is a screening estimate, typically within a few percentage points of methods like DEXA or hydrostatic weighing, but it can be off for very lean or very muscular people. Its real strength is consistency: measure the same way each time and it tracks change well, even if the absolute number is approximate.
Where exactly do I measure?
Neck: just below the larynx, tape sloping slightly down at the front. Waist: men at the navel, women at the narrowest point. Hips (women): around the widest part. Keep the tape snug but not compressing the skin, and stand relaxed without holding your breath.
What is a healthy body-fat percentage for a runner?
It varies by sex and individual. Many trained distance runners sit in the athletes or fitness bands, but lower is not always better — dropping too low harms hormones, immunity and bone health. Health and durability matter more than chasing a minimum; do not under-fuel to lower the number.
How is body fat different from BMI?
BMI uses only height and weight and cannot tell muscle from fat, so muscular runners can read as overweight. Body fat estimates the fat fraction directly, so it is more informative about composition. They answer different questions — use them together with the BMI calculator for a fuller picture.
Why do men and women use different formulas?
Women carry more essential fat and store it differently, so the Navy method adds the hip measurement and uses different constants for women. That is also why the healthy category bands sit several points higher for women than for men.
This is a screening estimate from body circumferences, not a precise measurement of body composition, and it can be several percentage points off for lean or muscular individuals. It is general information and education, not medical advice. Body-fat numbers should never drive extreme dieting; if you have concerns about weight, eating or body composition, speak with a qualified doctor or dietitian.