Why body fat matters more than BMI
BMI doesn't distinguish muscle from fat. A muscular athlete reads “overweight”; a sedentary thin person reads “healthy.” Body fat % tells you what your weight is actually made of — which is what's tied to metabolic health, hormone function, and disease risk.
How to measure correctly
- Waist: Stand relaxed, tape horizontal at navel level. Exhale normally — don't suck in.
- Neck: Just below the larynx (Adam's apple), tape sloping slightly down at the front.
- Hip (women only): Widest point around the buttocks, tape horizontal.
- Use a flexible, non-stretching tape. Take 3 measurements and average them.
The Navy formula
The US Navy method was developed by Hodgdon & Beckett (1984) as a fast, equipment-free body composition test for sailors. It uses log-transformed circumference measurements:
Men: 495 / (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450
Women: 495 / (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.22100·log₁₀(height)) − 450More accurate methods
For research-grade accuracy: DEXA scan (gold standard, ±1.5%), BodPod (±2.7%), or hydrostatic weighing. Skinfold calipers in trained hands are ±3.5%. Bioelectrical impedance (smart scales) are typically ±5–8% and very dependent on hydration. For tracking changes over time, pick one method and stick with it — the absolute number matters less than the trend.
References
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center, 1984.
- Heyward VH, Wagner DR. Applied Body Composition Assessment. Human Kinetics, 2nd ed.