The US Navy formula
The Navy method estimates body fat from height and circumference measurements. With all measurements in centimeters:
Men: BF% = 495 / (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450 Women: BF% = 495 / (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.22100·log₁₀(height)) − 450
If you enter inches, they are converted to centimeters first (1 inch = 2.54 cm). When you add your weight, fat mass = weight × BF% and lean mass = weight − fat mass.
How to calculate it by hand
Take a man who is 178 cm tall with a 38 cm neck, an 86 cm waist and a weight of 80 kg:
- Subtract the neck from the waist: 86 − 38 = 48.
- Take the base-10 logarithms: log₁₀(48) ≈ 1.6812 and log₁₀(178) ≈ 2.2504.
- Build the denominator: 1.0324 − 0.19077 × 1.6812 + 0.15456 × 2.2504 = 1.0324 − 0.3207 + 0.3478 ≈ 1.0595.
- Divide and subtract: 495 ÷ 1.0595 ≈ 467.20, and 467.20 − 450 = 17.2% body fat.
- Apply it to weight: fat mass = 80 × 0.172 ≈ 13.8 kg, lean mass = 80 − 13.8 = 66.2 kg.
17.2% puts him in the ACE Fitness range. For a woman, the steps are the same with the women's constants. A woman 165 cm tall with a 33 cm neck, 76 cm waist and 100 cm hips uses 76 + 100 − 33 = 143 and comes out at about 29.9% (Average). At 65 kg that is about 19.5 kg of fat and 45.5 kg of lean mass.
How to measure
- Neck: just below the larynx (Adam's apple), with the tape sloping slightly downward to the front.
- Waist (men): horizontally at the level of the navel.
- Waist (women): at the narrowest point of the natural waist.
- Hip (women): around the widest part of the hips and buttocks.
Measure on bare skin, relaxed, with the tape snug but not pressing into the skin. Taking each measurement two or three times and using the average makes the result more consistent.
ACE body fat categories
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Average | 25–31% | 18–24% |
| Obese | 32% and above | 25% and above |
Women's ranges are higher because women carry more essential fat linked to reproductive health. Compare your result with the column for your sex only.
Common mistakes
- Pulling in your stomach. Breathe out normally and let the waist relax, or the result will read low.
- Measuring the waist in the wrong place. Men measure at the navel, not at the narrowest point, which is the women's landmark.
- Pulling the tape too tight. A tape that dents the skin shortens every circumference.
- Comparing readings from different times of day. Measure at the same time, ideally in the morning before eating, so the trend is meaningful.