Body fat calculator — US Navy method
This calculator estimates your body fat percentage from a few tape-measure readings using the US Navy circumference method. Unlike BMI, which cannot tell muscle from fat, this method accounts for where the fat sits, making it far more useful for tracking body composition during a cut or recomposition. All you need is a soft tape measure.
Why the Navy method rather than BMI
BMI divides your weight by the square of your height and cannot distinguish a kilogram of fat from a kilogram of muscle. A powerlifter and a sedentary person of the same height and weight have the same BMI but wildly different body compositions. The US Navy method sidesteps this by measuring circumferences — the places on the body where fat accumulates — and using them in a regression formula that predicts the percentage of total body mass that is fat. It is not perfect (DEXA or hydrostatic weighing are more accurate), but it requires only a tape measure and is accurate within roughly 3–4 percentage points for most people.
How to measure correctly
Neck: Place the tape just below the larynx (Adam’s apple), keeping it perpendicular to the spine. Do not compress the skin.
Waist (men): Measure at the narrowest point, typically above the navel and below the ribcage, while relaxed and not holding your breath.
Waist (women): Measure at the navel level, not the narrowest point.
Hips (women only): Measure at the widest point, keeping the tape horizontal.
Measure in the morning before eating or exercising, and take the average of two or three readings for consistency.
How it works
The Navy formula works in inches, so metric inputs are divided by 2.54 first. It then applies a base-10 logarithmic regression that differs by sex:
- Men:
86.01 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76 - Women:
163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
The result is matched to the ACE (American Council on Exercise) category bands. Measure the neck below the larynx, the waist at the navel (women) or narrowest point (men), and the hips at their widest point (women).
Worked example
A man 70 in tall with a 15 in neck and 33.5 in waist:
86.01 × log₁₀(33.5 − 15) − 70.041 × log₁₀(70) + 36.76
= 86.01 × 1.267 − 70.041 × 1.845 + 36.76 ≈ 16.6% → Fitness category
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Above average | 25%+ | 32%+ |
Tracking progress over time
Body fat percentage from any circumference method should be tracked as a trend, not a single point-in-time reading. Measure every 2–4 weeks under the same conditions (time of day, hydration state) and note the direction of change. A decrease in waist measurement paired with a stable or increasing weight usually means you are replacing fat with muscle — exactly the change a recomposition aims for, and one that BMI would record as no change at all.
Everything is computed in your browser — none of your measurements leave your device.