Body Fat Calculator
Body-fat percentage using the US Navy circumference method
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What this calculator computes
The body-fat calculator estimates body-fat percentage using the US Navy method, a circumference-based formula developed in the 1980s for screening recruits and now widely used as a low-cost alternative to skinfold calipers, bioimpedance scales, and DEXA scans. The method requires only a tape measure: it takes neck circumference and waist circumference for men, and adds hip circumference for women, plus standing height. The Navy formula is empirically derived from regression against hydrostatic-weighing measurements and is accurate to about ±3 percentage points for most adults; it is less accurate at the extremes of the population (very lean physique athletes, very obese individuals, and bodybuilders with unusually large neck and shoulder development relative to waist). Body-fat percentage is a more meaningful measure of body composition than BMI, particularly for muscular individuals where BMI flags muscle as overweight, and for sedentary individuals who are at a healthy BMI but have high body-fat percentage (sometimes called "skinny-fat" or normal-weight obesity). Reference ranges from the American Council on Exercise are: essential fat 10–13% (women) / 2–5% (men); athletes 14–20% / 6–13%; fitness 21–24% / 14–17%; acceptable 25–31% / 18–24%; obese 32%+ / 25%+. **For informational purposes only — the Navy method is a screening estimate, not a clinical measurement. For weight-loss decisions, use the calculator's figure to track trends rather than as an absolute. Consult a healthcare professional for any concerns about body composition or weight.**
Calculator
The formula
Formula
Men: 86.010·log10(waist−neck) − 70.041·log10(height) + 36.76 Women: 163.205·log10(waist+hip−neck) − 97.684·log10(height) − 78.387
Worked example
When to use this calculator
Use this calculator when you need a no-equipment estimate of body-fat percentage and have access to a flexible tape measure. The most common use cases are tracking body-composition changes over time during a weight-loss or muscle-gain programme, screening fitness-level relative to ACE reference ranges, and getting a rough body-composition number when DEXA, hydrostatic, and bioimpedance methods are unavailable or impractical. The calculator is best used for tracking trends within an individual rather than for one-off absolute measurements; the ±3 percentage-point error band is too wide for clinical decisions but small enough to track weekly progress reliably if measurements are taken at the same time of day, on the same day of the week, and following the same fasted/post-void protocol. Take measurements with the tape parallel to the floor, snug but not compressing the skin, and at the standardised landmarks: neck just below the larynx, waist at the navel for men and at the narrowest point for women, hip at the widest point for women.
Common input mistakes
- Inconsistent tape placement between measurements. Body-fat calculators are very sensitive to small changes in waist measurement: a 1 cm error in waist circumference shifts the result by about 0.7 percentage points for men. Always measure at the same anatomical landmark (navel for men, narrowest point for women) and at the same time of day under the same conditions (fasted, post-void) for tracking comparability.
- Treating the Navy method's result as a clinical measurement. The Navy formula was empirically derived against hydrostatic weighing in a specific population and carries a ±3 percentage-point error band. It systematically under-predicts in very muscular subjects (where neck circumference is large relative to body fat) and over-predicts in tall, lean subjects. Use the figure as a trend tool, not an absolute measurement, and seek DEXA scanning if a precise number is required for a clinical or athletic-performance reason.
Frequently asked questions
How accurate is the Navy body-fat method?
The Navy circumference method is accurate to about ±3 percentage points compared to hydrostatic weighing for typical adults of moderate body composition. It under-predicts in very muscular subjects (whose neck circumference is large relative to actual fat mass) and over-predicts in very lean tall subjects. For tracking trends over time it is reliable; for absolute precision, DEXA, hydrostatic, or air-displacement (Bod Pod) measurements are more accurate but typically cost £80–200 per scan.
Where exactly should I measure?
Neck: just below the larynx (Adam's apple), tape parallel to the floor. Waist (men): horizontally at the navel. Waist (women): at the narrowest point of the torso, typically a few centimetres above the navel. Hip (women only): at the widest point of the hips/buttocks, tape parallel to the floor. Take all measurements first thing in the morning after using the toilet, in light clothing or underwear, with the tape snug but not compressing the skin.
What is a healthy body-fat percentage?
American Council on Exercise reference ranges: essential fat 10–13% women / 2–5% men; athletes 14–20% / 6–13%; fitness 21–24% / 14–17%; acceptable 25–31% / 18–24%; obese 32%+ / 25%+. Healthy ranges depend on age, sex, and ethnicity; the ACE ranges are a rough guide rather than a precise diagnostic threshold. Going below the essential-fat range is dangerous for both sexes; female competitors typically aim for 12–15%, well below the average healthy range.
How does this differ from BMI?
BMI uses only height and weight, classifying someone as underweight, normal, overweight, or obese based on a single number. It does not distinguish between muscle and fat, so a heavily-muscular individual is often classed as "overweight" despite very low body fat, and a sedentary individual with high body fat can fall in the "normal" BMI range. Body-fat percentage measures the actual proportion of fat tissue and is more useful for body-composition decisions, but is harder to measure than BMI.
Why are the formulas different for men and women?
Men and women store body fat differently. Men accumulate more fat around the waist (visceral and subcutaneous abdominal), while women store more around the hips and thighs and have higher essential fat in breast tissue and the reproductive system. The Navy formula adds hip circumference for women to capture this fuller distribution. Using the men's formula on a woman would systematically under-predict her body-fat percentage by 5–10 points because the hip storage is invisible to a waist-only measurement.