Body Fat Calculator (Navy Method)

How this is calculated

Men: 495 ⁄ (1.0324 − 0.19077·log₁₀(waist−neck) + 0.15456·log₁₀(height)) − 450

Women add the hip measurement inside the first logarithm. Worked example: male, 180 cm, neck 38, waist 85 → about 15% — the fit band. The tape cannot see where your fat actually sits, which is why the honest claim is direction-tracking within ±3%, not lab precision. For clinical numbers, a DEXA scan is the reference. Not medical advice.

What a tape measure can and cannot see

The circumference method on this page comes from a study of US Navy personnel and estimates body fat from neck, waist and — for women — hip measurements, corrected for height. It is free, repeatable at home, and accurate to roughly three to four percentage points against a laboratory reference for people whose build resembles the sample it was derived from. That is genuinely useful and clearly not precise. If your result says 22%, the honest reading is "probably somewhere between 18 and 26".

The method works because abdominal circumference correlates strongly with total fat, and neck circumference correlates with lean mass. It fails in the cases where that correlation breaks: very muscular necks make lean people read leaner still, and a build that carries fat somewhere other than the abdomen reads too low. Body-fat estimates for children and adolescents are not valid at all.

Measuring so the number means something

Technique matters more than method here. Measure first thing in the morning, before eating or drinking, after using the bathroom, standing relaxed rather than braced. Keep the tape level and snug enough to stay put without compressing the skin. Take the waist at the navel unless the instructions on the page say otherwise, and the neck just below the larynx with the tape sloping slightly downward at the front. Take each measurement three times and use the middle value.

Consistency beats accuracy. A method with a systematic two-point error, applied identically every month, tracks your actual change perfectly — and change is what you want. Switching between methods, or between morning and evening, introduces swings larger than the change you are trying to observe.

How the methods compare

Method	Typical error	Cost
DEXA scan	±1–2%	Clinic appointment
Hydrostatic weighing	±2%	Specialist lab
Skinfold calipers (trained)	±3%	Low, needs skill
Tape circumference (this page)	±3–4%	Free
Bathroom bioimpedance scale	±5–8%	Low

Bioimpedance scales deserve their reputation for volatility: they pass a current through the body and infer composition from resistance, which hydration state moves substantially. The same person can read three points apart before and after a glass of water, which is why a scale that reports body fat to one decimal place is displaying precision it does not have.

Reference ranges, and the floor you should not cross

Commonly cited ranges put essential fat at about 2–5% for men and 10–13% for women, athletic ranges at 6–13% and 14–20%, fitness at 14–17% and 21–24%, and average at 18–24% and 25–31%. Women carry more essential fat for reasons of reproductive physiology, which is why comparing a man's and a woman's percentage directly is meaningless.

The lower bound is a real floor rather than a stretch goal. Sustained very low body fat is associated with hormonal disruption, loss of menstruation, bone-density loss, poor immune function and impaired recovery; the single-digit figures visible in competition photographs are held for days, not seasons. For almost everyone, a stable figure in the fitness range with good strength and good sleep is a better target than a lower number reached by suffering.