What the body fat calculator measures
Body fat percentage is the share of your total weight that is fat tissue, as opposed to muscle, bone, organs and water. Unlike BMI, which only relates weight to height, body fat percentage distinguishes a lean, muscular person from someone of the same weight carrying more fat. This calculator uses the US Navy circumference method, which estimates body fat from a few tape measurements — height, neck and waist for everyone, plus hips for women — and needs no special equipment.
The formula
Men: BF% = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
Women: BF% = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
(all measurements in inches)The method was developed by the Naval Health Research Center and validated against underwater weighing. It works because the waist-to-neck difference tracks abdominal fat well, while height normalises for frame size. The calculator converts metric inputs to inches before applying the equations.
How to measure correctly
- Neck: just below the larynx (Adam's apple), tape sloping slightly downward to the front. Keep it snug but not compressing the skin.
- Waist (men): horizontally at the navel, at the end of a normal exhale, without pulling in.
- Waist (women): at the narrowest point of the torso, usually just above the navel.
- Hips (women): at the widest point of the buttocks, tape level all the way round.
- Height: without shoes, standing straight against a wall.
Measure two or three times and use the average. Being one inch off on the waist shifts the result by roughly 2–3 percentage points, so consistency matters more than precision if you are tracking change over time — same time of day, same tape, same posture.
Worked example
A man who is 70 in tall with a 34 in waist and a 15.5 in neck:
BF% = 86.010 × log₁₀(34 − 15.5) − 70.041 × log₁₀(70) + 36.76
= 86.010 × 1.2672 − 70.041 × 1.8451 + 36.76
≈ 108.99 − 129.23 + 36.76 ≈ 16.5%At 16.5% he falls in the “fitness” range for men. If he weighs 175 lb, that is about 29 lb of fat and 146 lb of lean mass.
Body fat ranges
| 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%+ | 25%+ |
These bands come from the American Council on Exercise and are widely used as a reference. Women naturally carry more essential fat for hormonal and reproductive function, which is why every band sits higher. Dropping below essential levels is harmful, not a goal.
Accuracy compared with other methods
| Method | Typical error | Cost / access |
|---|---|---|
| DEXA scan | ±1–2% | Clinic, $50–150 per scan |
| Hydrostatic weighing | ±1.5–2.5% | University labs |
| Bod Pod (air displacement) | ±2–3% | Some gyms and clinics |
| Skinfold calipers (trained tester) | ±3–4% | Cheap, technique-dependent |
| US Navy tape method | ±3–4% | Free, tape measure only |
| Bioimpedance scale | ±4–8% | Consumer scales; varies with hydration |
The Navy method is accurate enough to place most people in the right category and to track a trend, and it is considerably more reliable than a typical bathroom bioimpedance scale. It tends to under-estimate for very muscular individuals with thick necks and over-estimate for people who carry fat mainly on the hips and thighs rather than the waist.