Body Fat Calculator: U.S. Navy Method With Fitness Categories
Body fat percentage tells you what scales and BMI cannot: how much of your weight is fat versus lean tissue. This free body fat calculator uses the U.S. Navy circumference method — validated against lab scans and accurate to within about 3 percentage points for most adults — using nothing more than a flexible tape measure.
How the Body Fat Calculator Works
The Navy method infers body fat from the relationship between your height and the circumference of your neck and waist (plus hips for women). For men:
%BF = 495 / (1.0324 − 0.19077 × log(waist − neck) + 0.15456 × log(height)) − 450
For women:
%BF = 495 / (1.29579 − 0.35004 × log(waist + hip − neck) + 0.221 × log(height)) − 450
All measurements are in centimeters; the calculator converts from inches automatically if you prefer imperial units.
How to Measure Correctly
Accuracy lives or dies on measurement technique:
- Neck — measure just below the larynx (Adam's apple), tape sloping slightly downward toward the front
- Waist (men) — at the navel, relaxed, after a normal exhale
- Waist (women) — at the narrowest point of the torso
- Hips (women) — the widest point around the glutes
- Keep the tape snug against skin without compressing it, and take each measurement 2–3 times, using the average. Measure in the morning before food and drink for the most consistent trend data.
Worked Example
A 5'10" (178 cm) man weighing 185 lb (84 kg) with a 15 in (38 cm) neck and 36 in (91.4 cm) waist:
- waist − neck = 91.4 − 38 = 53.4 cm
- log terms: log(53.4) = 1.727; log(178) = 2.250
- denominator = 1.0324 − 0.19077 × 1.727 + 0.15456 × 2.250 = 1.0503
- %BF = 495 / 1.0503 − 450 = ≈ 21.4%
That places him in the "Average" category, with roughly 18 kg of fat mass and 66 kg of lean mass — both reported by the calculator automatically.
What Your Number Means: ACE Categories
| Category | Men | Women | |---|---|---| | Essential fat | 2–5% | 10–13% | | Athletes | 6–13% | 14–20% | | Fitness | 14–17% | 21–24% | | Average | 18–24% | 25–31% | | Obese | 25%+ | 32%+ |
Women carry higher essential fat for hormonal and reproductive function — a woman at 15% is as lean as a man at 5%. For general health, the "fitness" band is a realistic, sustainable target for most people.
Why Body Fat Beats Scale Weight
Two people can weigh 180 lb while one carries 15% fat and the other 30% — identical BMI, very different health profiles. Body fat percentage is the number that actually predicts metabolic health markers, and it changes even when scale weight stalls (a classic "recomposition": losing fat while gaining muscle). Track body fat monthly under identical conditions and you will see progress the mirror and scale both miss. If you want the calorie targets that match your new lean mass, follow up with our TDEE calculator.
This tool estimates body fat for healthy adults and is not a medical measurement. Very muscular individuals may see overestimates; DEXA remains the clinical gold standard.