What is BMI?
BMI (Body Mass Index) is a weight-for-height index used as a screening tool for underweight, healthy weight, overweight, and obesity in adults. It is not a diagnosis by itself.
Formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = (weight (lb) × 703) ÷ height (in)²
WHO adult categories
| Category | BMI |
|---|---|
| Underweight | < 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity class I | 30.0 – 34.9 |
| Obesity class II | 35.0 – 39.9 |
| Obesity class III | ≥ 40.0 |
Important caveats
- Doesn’t separate fat vs muscle or bone, or where fat is stored.
- Athletes and people with high muscle mass can show a high BMI without excess fat.
- Age, sex, and ethnicity matter; Asian populations often use lower risk thresholds (around 23 and 27.5).
- For children and teens, use age- and sex-specific percentiles — not adult cutoffs.
- Best used with waist measures, labs, and clinical context — not alone.
Sources: WHO obesity fact sheet · WHO NLIS BMI · CDC About BMI