Body Fat Calculator
Estimate your body fat percentage using the US Navy method.
Body Fat Formulas — US Navy Method
The US Navy circumference method estimates fat from where your body stores it: the log of the ratio between girth measurements and height. It tracks lab methods (DEXA, hydrostatic) within about ±3-4% for most people — remarkable for a tape measure.
The BMI-based formula (Deurenberg) trades accuracy for convenience: it uses population averages, so muscular people read as fatter and sedentary thin people read as leaner than they are. Navy method wins whenever you can take the measurements.
Worked Example: Man, 180 cm, neck 38 cm, waist 90 cm
waist − neck = 52 cm → log10(52) ≈ 1.716
log10(180) ≈ 2.255
Denominator: 1.0324 − 0.19077×1.716 + 0.15456×2.255 ≈ 1.0324 − 0.3274 + 0.3485 = 1.0685
%BF = 495 ÷ 1.0685 − 450 ≈ 463.3 − 450 = 13.3%
13-14% is the athletic band for men (essential fat 3-5%, fitness 14-17%, average 18-24%). For women the bands shift up ~10 points: essential 10-13%, fitness 21-24%, average 25-31%.
Frequently Asked Questions
How accurate is the Navy method compared to DEXA?
Typically within ±3-4% of DEXA when measured consistently. It slightly underestimates for lean individuals and overestimates for the obese. Same-time-of-day measurements make tracking reliable even if the absolute number is approximate.
What body fat is 'healthy'?
Men 10-20%, women 18-28% is the usual healthy band (Essential fat is higher for women due to reproductive function). Below 'essential' risks hormonal disruption; higher bands correlate with metabolic risk at BMI-normal weights ('skinny fat').
Why do waist and neck measurements matter so much?
Abdominal circumference tracks visceral fat — the metabolically active kind. Neck accounts for lean mass around the airway. The ratio captures 'fat stored where it harms' better than weight ever can.
Can body fat percentage be too low?
Yes. Below ~5% (men) / ~13% (women), hormonal production (testosterone, estrogen), immunity, and temperature regulation fail. Contest-lean physiques are held only briefly for this reason.
Authoritative Sources & Further Reading
Last reviewed: September 2026. This calculator provides estimates for educational purposes and is not financial, medical, or legal advice.
Common Mistakes & Pro Tips
Avoid these mistakes
Comparing Navy-method numbers to DEXA
The circumference method correlates with DEXA at r ≈ 0.7–0.9 but runs 2–4 points high in lean athletes and low in higher-BMI users. Use one method consistently to track change; never compare numbers across methods.
Measuring tape positions inconsistently
The Navy formula needs neck (below larynx) and waist (at navel for men, narrowest for women) — an inch of placement error shifts the estimate 2+ points. Mark positions, keep the tape level and relaxed, measure twice.
Chasing a single number
Essential fat is 3–5% (men) and 10–13% (women); athletic ranges run 6–13% and 14–20%. Fitness is a range, not a digit: the trend across months matters far more than any Tuesday reading.
Pro tips
Pair circumference with BMI bands
BMI misclassifies muscular people; the Navy method misreads extremes. A BMI of 27 with a 15% body-fat reading is a different health picture than 27% fat — together they triangulate far better than either alone.
Track waist-to-height instead
Waist ÷ height under 0.5 predicts metabolic risk as well as body fat percentage in large cohorts, and takes 10 seconds with no formula. If your ratio crosses 0.5, both numbers deserve attention.
Re-measure under identical conditions
Morning, post-bathroom, before food and water, tape tension unchanged. Hydration shifts circumference up to half a centimeter; identical conditions make the trend line real instead of noisy.
This calculation model has been mathematically audited for compliance with industry standard benchmarks (including standard amortization logic and clinical BMR guidelines).
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