Body Fat Calculator
No scan, no fancy scale — a tape measure and the U.S. Navy formula: body-fat estimate with category, exact measuring instructions, and its honest ±3–4% error band attached.
Spotted a mistake or have an idea? — errors get fixed and credited publicly.
Last reviewed: July 20, 2026 · Formulas verified by automated tests before every release.
How can I estimate body fat at home?
Use the U.S. Navy circumference method: neck, waist (plus hips for women) and height in a logarithmic formula. Example: 178 cm tall, 38 cm neck, 85 cm waist ≈ 16% — the Fit range. Expect ±3–4% versus lab methods; measure the same spots each time and trust the trend more than any single reading.
The formula and why the tape wins for trends
men: 495 ÷ (1.0324 − 0.19077·log₁₀(waist−neck) + 0.15456·log₁₀(height)) − 450
Lab scans beat the tape on any single day; the tape beats everything on consistency per dollar — same spots, same morning, once a month, and the trend line is yours for free. Pair the composition picture with maintenance calories and a macro split to act on it; keep the BMI calculator for the different, screening-level question.
Sources, assumptions and limitations
- Formula: U.S. Navy circumference equations (Hodgdon & Beckett, Naval Health Research Center, 1984), metric log₁₀ form; ACE-style category bands
- Assumptions: Correct measuring spots (instructions on-page); adults. Accuracy band ±3–4 pp vs DEXA stated by design; edges (very lean / very muscular) err larger.
- Not included: Clinical body-composition assessment, athletes' certification cutoffs, children. Educational estimate — for decisions that matter medically, a clinician beats a tape.
- Last reviewed: July 30, 2026 by the CalcNotebook team
Frequently asked questions
How do I calculate body fat with a tape measure?
The U.S. Navy method: measure neck and waist (plus hips for women) and height, then apply a logarithmic formula. A man at 178 cm with a 38 cm neck and 85 cm waist lands around 16% — Fit range. All you need is a soft tape and consistent measuring spots.
How accurate is the Navy method?
Honestly: ±3–4 percentage points versus a DEXA scan for most people, with larger errors at the very lean and very muscular extremes. Its real strength is trend tracking — the same tape at the same spots on the same morning makes the direction of change far more reliable than any single number.
Where exactly do I measure?
Neck: just below the larynx, tape sloping slightly down at the front. Waist: men at navel level, women at the narrowest point. Hips (women): at the widest point. Relaxed, after a normal exhale, tape snug but not compressing skin — and always the same spots.
Body fat % or BMI — which should I track?
They answer different questions. BMI is a population screening ratio that can't tell muscle from fat; body fat % estimates composition, which is usually what people actually mean. A muscular person can be "overweight" by BMI at 12% body fat. Track body fat for composition, keep BMI for what it is — a rough screen.