Body fat calculator

Hodgdon & Beckett (1984) regressed body-fat percentage from hydrostatic weighing onto a small set of body circumferences. The Navy adopted the result as its body-composition screening tool — it shows up in the Department of Defense Instruction 1308.3 (and is a building block of the Army's AR 600-9):

• 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

Inputs are in inches. We accept cm and convert internally so you don't have to.

Tape placement is the single biggest source of error. Use a flexible (cloth or fibreglass) tape, snug but not compressing skin:

• Neck — just below the larynx (Adam's apple), tape angled slightly down at the front. Stand relaxed; do not flex the neck.
• Waist — men: horizontally at the navel. Women: at the narrowest point (usually 1–2 inches above the navel). Breathe normally; don't suck in or push out.
• Hip (women only) — horizontally at the widest point of the buttocks, feet together.
• Height — first thing in the morning, no shoes, heels and back against a wall.

Measure each value twice and average. If the two readings differ by more than ½ inch / 1.25 cm, you're moving the tape too much.

The American Council on Exercise (ACE) publishes the most commonly cited category bands:

• Essential fat (M 2–5% / F 10–13%) — the bare minimum for normal physiological function. Below this, hormonal, immune and reproductive systems suffer.
• Athletes (M 6–13% / F 14–20%) — typical for trained endurance and strength athletes during competition.
• Fitness (M 14–17% / F 21–24%) — visible muscle definition; what most regular gym-goers are aiming for.
• Average (M 18–24% / F 25–31%) — the general population.
• Obese (M ≥ 25% / F ≥ 32%) — elevated metabolic-disease risk; aim to reduce gradually.

Validation studies place the Navy method at ±3–4 absolute percentage points vs DEXA scan, the research reference standard. That's better than BMI for body composition and roughly comparable to skinfold calipers in untrained hands. It's worse than DEXA, BodPod (air-displacement plethysmography) and hydrostatic weighing.

The accuracy is in change over time rather than the absolute number — measurement error tends to be consistent for a given person if they always measure the same way. To track progress:

• Measure at the same time of day (morning, fasted is most stable).
• Always use the same tape and the same reference points.
• Re-measure every 2–4 weeks. Daily fluctuations are mostly hydration, not body composition.