Body fat percentage — the proportion of your total weight that's fat tissue — is a much more meaningful health metric than weight or BMI alone. Two people can weigh the same but have completely different body compositions, fitness levels, and health risks.
What's a Healthy Body Fat Percentage?
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletic | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
Women naturally carry more essential fat due to hormonal and reproductive functions.
Method 1: US Navy Tape Measure Method
Accurate to within 3–4% and requires only a tape measure. Different formulas for men and women.
For Men (measure neck and abdomen):
Body fat % = 86.010 × log₁₀(abdomen − neck) − 70.041 × log₁₀(height) + 36.76
All measurements in centimetres.
For Women (measure neck, waist, and hips):
Body fat % = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
How to measure:
- Neck: Measure at the narrowest point (just below the larynx for men)
- Abdomen (men): At the navel level, relaxed (not sucked in)
- Waist (women): At the narrowest point, usually just above the navel
- Hips (women): At the widest point of the buttocks
Example — Male: Height 180 cm, neck 38 cm, abdomen 90 cm
Body fat % = 86.010 × log₁₀(90 − 38) − 70.041 × log₁₀(180) + 36.76
= 86.010 × log₁₀(52) − 70.041 × log₁₀(180) + 36.76
= 86.010 × 1.716 − 70.041 × 2.255 + 36.76
= 147.6 − 157.9 + 36.76
≈ 26.4%
Method 2: Skinfold Calipers
Skinfold calipers pinch and measure subcutaneous fat at specific sites on the body. More time-consuming but can be accurate to 3–4% when done consistently.
Common sites (7-site Jackson-Pollock protocol): Chest, abdomen, thigh, triceps, suprailiac, subscapular, midaxillary
Durnin-Womersley 4-site (bicep, tricep, subscapular, suprailiac): This is commonly used in UK clinical settings.
Skinfold accuracy depends heavily on technique — the same person measuring consistently over time gives better trend data than absolute numbers.
Method 3: BMI-Based Estimate
Body fat can be roughly estimated from BMI:
Deurenberg formula:
Body fat % = (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4
Where sex = 1 for male, 0 for female.
Example — 35-year-old man, BMI 24:
= (1.20 × 24) + (0.23 × 35) − (10.8 × 1) − 5.4
= 28.8 + 8.05 − 10.8 − 5.4
= 20.65%
Limitations: Less accurate than tape or calipers. Doesn't account for athletic build (high muscle mass).
Method 4: DEXA Scan
Dual-Energy X-ray Absorptiometry (DEXA) is considered the gold standard for body composition measurement.
How it works: A low-dose X-ray machine scans your body and distinguishes between fat, lean mass, and bone mineral density with high precision.
Accuracy: ±1–2% — the most accurate non-laboratory method.
Cost: £50–200 per scan at specialist clinics. Not typically available from a GP.
Best for: Baseline measurement and tracking progress over 3–6 months.
Method 5: Bioelectrical Impedance (BIA)
Scales and handheld devices that send a small electrical current through the body to estimate body fat.
How it works: Fat conducts electricity poorly; muscle (with water) conducts it well. The resistance measured gives an estimate of body fat.
Accuracy: ±3–8% — highly variable depending on hydration, time of day, and device quality.
Best for: Rough tracking of trends, not absolute numbers.
Comparison of Methods
| Method | Accuracy | Cost | Convenience |
|---|---|---|---|
| Navy tape | ±3–4% | Free | Easy |
| Skinfold calipers | ±3–5% | £10–30 | Moderate |
| BMI formula | ±5–7% | Free | Easy |
| BIA scales | ±3–8% | £30–200 | Very easy |
| DEXA scan | ±1–2% | £50–200 | Specialist clinic |
| Hydrostatic weighing | ±1–2% | £50–100 | Rare |
Which Method Should You Use?
For most people: The Navy method is the best balance of accuracy and convenience. Measure consistently — same time of day, same person measuring.
For athletes: DEXA scan once or twice a year for precise tracking.
For daily tracking: BIA scales are fine for trend data, as long as you weigh under consistent conditions (same time, fasted, after using the bathroom).
Why BMI Misses the Point
Two people with a BMI of 25 can have vastly different body compositions:
- Person A: BMI 25, body fat 12% — lean and muscular
- Person B: BMI 25, body fat 28% — "skinny-fat" — low muscle, higher fat
BMI gives no information about this distinction. Body fat percentage does.