How it works
U.S. Navy (men): %fat = 495 ÷ (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450
The Navy method regresses body-fat percentage onto logarithms of circumferences measured in inches. For men, the waist-minus-neck difference and height feed the log₁₀ terms — weighted 0.19077 and 0.15456 — inside the published constants 495, 1.0324, and 450; a man taping 34 in at the waist, 16 in at the neck, and 68 in tall lands near 10% body fat. Women add the hip girth and use their own coefficients, reflecting different essential-fat physiology. Centimeter entries are converted to inches before the math runs, and tape placement drives accuracy more than anything else: neck just under the larynx, waist at the navel for men or the narrowest point for women, hips at the widest point. The resulting percentage is binned into the ACE fitness categories, which sit several points higher for 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
What measurements does the body fat calculator need?
Height, neck, and waist for men; height, neck, waist, and hips for women — all in centimeters or inches, taken with a flexible tape. No scale weight is involved at all: the Navy formula works purely from circumferences, which is what makes it doable at home in two minutes.
How close is a tape estimate to a DEXA scan?
Usually it lands within a few points of lab methods, with bigger misses at the extremes — very lean or heavily muscled bodies fool circumference math the most. Its real strength is repeatability: measure the same spots under the same conditions each month and the trend is trustworthy even when the absolute number is soft.
How do I take the measurements correctly?
Stand relaxed, breathe normally, and keep the tape level and snug without denting the skin. The neck goes just below the larynx; men take the waist at navel height while women use the narrowest point; the hip tape wraps the widest part. Repeat at the same time of day for consistent tracking.
What body-fat percentage should a distance runner target?
The ACE athlete band is 6–13% for men, and women sit roughly 8–10 points higher, but a target is personal — plenty of strong racers live in the fitness band, and pushing toward essential-fat territory damages hormones, immunity, and bone. Never under-fuel to chase a lower reading.
Why use this instead of BMI?
BMI weighs the whole package without asking what it is made of; the Navy estimate splits fat from lean mass, so a muscular runner flagged as overweight by BMI can show a perfectly athletic fat percentage here. The two numbers together give more context than either alone.
Why do women measure hips when men do not?
Female physiology stores more essential fat, distributed differently, so the Navy researchers fitted a separate equation for women that needs the hip girth and its own constants. The category bands shift upward to match — a healthy female percentage would read as very lean on the male scale.
A circumference formula estimates composition rather than measuring it, and lean or muscular builds can land several points from a lab value. Follow the trend, not one reading, and never let the number push you toward extreme dieting — concerns about weight or eating belong with a doctor or dietitian, not a calculator.