What BMI tells you
Body mass index is weight (kg) ÷ height (m)². It is a quick screening number for adults, not a diagnosis: it does not tell fat from muscle, so athletes may show as overweight, and older people may carry more fat at the same BMI.
WHO categories for adults
- Below 18.5: underweight (below 16 is severe thinness)
- 18.5 – 24.9: normal weight
- 25 – 29.9: overweight
- 30 and above: obese (class I 30–34.9, class II 35–39.9, class III 40+)
For South Asian adults, health bodies such as the WHO expert consultation suggest that risk rises at lower BMIs (overweight from about 23 and obese from about 27.5). For children and teenagers, BMI must be compared with growth charts for age and sex.