How it works
What "body fat percentage" actually measures
Your total body mass is the sum of fat mass (subcutaneous + visceral fat) and lean mass (muscle, bone, organs, water). Body fat percentage is simply fat mass divided by total mass. It's a more useful indicator of metabolic health than BMI, because two people of identical BMI can carry very different amounts of muscle.
This body fat calculadora uses the US Navy tape-measure formula, which a RAND Corporation study validated against hydrostatic weighing with a standard error of around 3–4 percentage points — not as tight as a DEXA scan, but accessible to anyone with a soft tape measure.
The US Navy formulas, exact
All measurements in centimetres. If you prefer inches, multiply your cm figure by 0.3937.
- 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
How to measure correctly
Waist — horizontal, at the narrowest point above the navel, breathing out normally. Don't suck in.
Neck — just below the larynx, with the tape perpendicular to the long axis of the neck.
Hip (women only) — the widest point across the buttocks, feet together.
Height — without shoes, heels and shoulders to a wall.
Measure three times and average. First-time users typically over-tighten the tape, which shows up as artificially low readings.
A worked example
Example: a 180 cm tall man with a waist of 84 cm and neck of 39 cm.
waist − neck = 84 − 39 = 45 cm.
log₁₀(45) = 1.6532; log₁₀(180) = 2.2553.
%BF = 86.010 × 1.6532 − 70.041 × 2.2553 + 36.76 = 142.17 − 157.95 + 36.76 = 20.98 %.
This sits at the upper edge of the "acceptable" range for adult men, just above what is often called "average fitness".
Healthy ranges by age and sex
The American Council on Exercise categories below are the most widely quoted. Age adjusts the bars: at 50+ even an active adult typically sits 4–5 points higher than at 25.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Acceptable | 18–24% | 25–31% |
| Obese | ≥ 25% | ≥ 32% |
Limitations of any single reading
- The US Navy formula assumes typical fat distribution — it over-estimates in very lean athletes with small waists and under-estimates in abdominal-obese adults.
- Hydration swings skew impedance-based scales (BIA) by ± 3% between morning and evening. Tape-measure methods are steadier.
- DEXA is the gold standard but costs £60–£200 in the UK and delivers a very small radiation dose.
- Trend matters more than the absolute number. Track the same method, same tape, same time of day across weeks.
Works well with
Body-fat figures are most useful alongside other data points — these companion calculadoras give you the rest of the picture.
- **BMI calculadora** — sense-check against height-weight.
- **BMR calculadora** — resting metabolic rate.
- **TDEE calculadora** — total daily calories for your activity.
- **Waist-to-hip ratio** — complementary cardiometabolic marker.
How we keep this accurate
The calculadora uses the exact US Navy regression coefficients as published in the Department of Defense Directive 1308.3. See our editorial policy and corrections policy, and please email us if you spot a bug — we publish a change log of every material update.