Body Mass Index (BMI) & Its Limits
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Weight in kg divided by height in metres squared — a fast population screen for weight status that says nothing about body fat distribution and mislabels muscular people.
Detail
BMI = body mass (kg) ÷ height (m)² (or 703 × lb ÷ in²). It is a cheap, universal screening index for weight status at the population level.
WHO adult categories:
| Category | BMI (kg/m²) |
|---|---|
| Underweight | < 18.5 |
| Normal weight | 18.5–24.9 |
| Overweight | 25.0–29.9 |
| Obese (Class I) | 30.0–34.9 |
| Obese (Class II) | 35.0–39.9 |
| Obese (Class III) | ≥ 40.0 |
Limits (important): BMI does not measure body fat and cannot distinguish fat from muscle or bone — muscular athletes are frequently misclassified as "overweight/obese," while "normal-weight obesity" (high fat, low muscle) is missed. It ignores fat distribution (visceral vs. peripheral), and cutoffs differ by ethnicity (e.g., lower obesity thresholds recommended for some Asian populations). It is best used alongside waist circumference and a direct body-fat method, not alone. See overweight-vs-overfat.
Key facts
- BMI = kg/m² (or 703 × lb/in²).
- WHO: normal 18.5–24.9, overweight 25–29.9, obese ≥30 (Class I 30–34.9, II 35–39.9, III ≥40).
- Does not measure body fat; misclassifies muscular individuals; ignores fat distribution.
- Ethnicity-specific cutoffs exist (lower for some Asian populations).
- Use with waist circumference + a body-fat method, not in isolation.
Connections
- waist-circumference / waist-to-hip-ratio — add fat-distribution info BMI lacks.
- body-composition-overview — places BMI among direct methods.
- obesity-thresholds / overweight-vs-overfat — the nutrition-side context.
Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.