Body Fat Calculator

Estimating body fat with the US Navy method

The US Navy circumference method estimates body fat percentage from a few tape measurements — height, neck, and waist for men, plus hip for women — using a formula fitted to hydrostatic-weighing data. It works because fat distributes in predictable places: a larger waist relative to neck and height signals a higher fat percentage. No calipers, scales, or scans are needed.

Once body fat percentage is known, splitting body weight into fat mass and lean body mass is simple: fat mass is weight × body-fat %, and lean mass (muscle, bone, organs, water) is the remainder. Tracking lean mass over time is useful when dieting — the goal is usually to lose fat mass while holding lean mass roughly steady.

Accuracy: the Navy method is typically within 3–4 percentage points of a DEXA scan for people of average build, and less accurate at the extremes — very lean athletes and people carrying a lot of fat. Measurement technique is the main source of error. Measure at the same time of day, tape snug but not compressing the skin, level around the body, and relaxed (do not suck in). Take each measurement twice and average.

Category ranges differ by sex because women carry more essential fat. Rough bands: for men, 2–5% essential, 6–13% athletic, 14–17% fitness, 18–24% average, 25%+ above average; for women, add roughly 8–10 points to each band. These are general references, not clinical thresholds — use the trend, not a single reading, and consult a professional for a body-composition assessment.

All measurements in centimetres.
  men:   %fat = 495 / (1.0324 − 0.19077·log10(waist − neck) + 0.15456·log10(height)) − 450
  women: %fat = 495 / (1.29579 − 0.35004·log10(waist + hip − neck) + 0.22100·log10(height)) − 450
Fat mass = weight × %fat ;  Lean mass = weight − fat mass

Worked example

A man 5 ft 10 in tall weighing 176 lb, with a 15-inch neck and a 34-inch waist, gets an estimate of 17.4% body fat — about 30.7 lb of fat mass and 145.3 lb of lean mass. A woman 165 cm tall with a 33 cm neck, 76 cm waist and 100 cm hips gets 29.9%.

Switching between imperial and metric converts the measurements you have entered, so you can measure in whichever unit your tape shows.

Getting consistent measurements

• Measure in the morning before eating, relaxed and not sucking in your stomach.
• Men: waist horizontally at the navel. Women: waist at the narrowest point and hips at the widest point of the buttocks.
• Neck: just below the larynx (Adam’s apple), with the tape sloping slightly downward at the front.
• Take each measurement two or three times and use the average; a 1 cm change in waist moves the estimate by roughly one percentage point.

Because the Navy method depends mostly on the waist-to-neck difference, it tracks changes in one person well even when the absolute number is a few points off. Compare yourself with your own previous readings rather than with someone else’s.

Other ways to measure body fat

• DEXA scan: the clinical reference, usually within 1–2 percentage points, and it also measures bone density.
• Bioimpedance scales: convenient but sensitive to hydration — readings can swing several points between morning and evening.
• Skinfold calipers: accurate in experienced hands, much less so when self-measured.