Body fat percentage is one of the most useful fitness numbers, but actually measuring it can be confusing. Five common methods give five slightly different answers — and the gap between the cheapest and the most accurate is large enough to matter.
This guide walks through every common method, the math behind each, and which one to pick based on your accuracy needs and budget.
Method 1 — U.S. Navy circumference method
Developed by the U.S. Navy for fitness testing, this method needs only a tape measure. For men: BF% = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76. For women: BF% = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387 (all measurements in cm).
Accuracy
±3–4% versus DEXA when measurements are taken carefully.
Cost
Free. Needs only a flexible measuring tape.
Method 2 — skinfold calipers
Calipers pinch the fold of skin and subcutaneous fat at 3, 5, or 7 sites (triceps, subscapular, suprailiac, abdomen, thigh, chest, midaxillary). The Jackson–Pollock equations turn the sum into body fat percentage. Accuracy: ±3–5%, highly dependent on the technician. Cost: ~$10–50 for calipers, plus a learning curve.
Method 3 — bioelectrical impedance (BIA)
Smart scales and hand-held devices send a tiny electrical current through the body and measure resistance. Fat conducts poorly; lean tissue conducts well. Accuracy: ±3–5% for clinical-grade devices, ±5–8% for consumer smart scales. Strongly affected by hydration, recent meals, and exercise. Cost: $30–500.
Method 4 — DEXA scans
Dual-energy X-ray absorptiometry uses two X-ray beams to measure bone, fat, and lean tissue separately. The current gold standard for body composition outside research labs. Accuracy: ±1–2%. Cost: $50–150 per scan in most cities.
Method 5 — hydrostatic weighing
Underwater weighing measures body density and infers fat from the difference. Highly accurate (±1.5%) but requires a specialised tank and trained staff. Cost: $40–100, mostly available at university labs.
Worked example
A man, 178 cm, with neck = 38 cm and waist = 88 cm. Plug into the male Navy formula: 86.010 × log10(88 − 38) − 70.041 × log10(178) + 36.76 = 86.010 × 1.699 − 70.041 × 2.250 + 36.76 = 146.13 − 157.59 + 36.76 ≈ 25.3%.
A woman, 165 cm, with neck = 34 cm, waist = 76 cm, hips = 96 cm. 163.205 × log10(76 + 96 − 34) − 97.684 × log10(165) − 78.387 = 163.205 × 2.140 − 97.684 × 2.218 − 78.387 ≈ 27.4%.
Common mistakes and misconceptions
- Measuring once and trusting it absolutely. Use 3 measurements averaged or measure on consecutive mornings.
- Measuring the waist at the wrong spot. Use the navel for the Navy method, not the narrowest point.
- Hopping between methods. Navy and BIA can disagree by 3–6%. Track trends with one method.
- BIA after a workout or coffee. Dehydration inflates apparent body fat by 1–3% in BIA devices.
Tips for accurate body fat measurement
- Measure first thing in the morning after using the bathroom, before food or water.
- Stand relaxed with arms at sides. Don't suck in the waist or flex.
- Use the same tape and pull it snug but not tight enough to indent the skin.
- Always take 3 readings and use the average if any differ by more than 1 cm.
Related questions
Which body fat measurement method is most accurate?
Why do my scale and the Navy calculator disagree?
Can I calculate body fat without a tape measure?
How often should I measure body fat?
Is the Navy method less accurate for very lean or very heavy people?
Keep reading on this site
- Navy body fat calculator — the formula applied for you
- TDEE & Body Fat Calculator — calorie burn plus body composition
- What is body fat percentage? — the meaning of the number
- What is lean body mass? — the other side of the calculation