What BMI measures
Body mass index divides weight in kilograms by height in metres squared. It was designed in the nineteenth century to describe populations, not individuals, and it remains useful for exactly that reason: it is quick, needs no equipment, and correlates reasonably well with body fat across large groups.
The WHO categories for adults
| Category | BMI |
|---|---|
| Underweight | below 18.5 |
| Healthy 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 and above |
Where BMI gets it wrong
BMI cannot tell muscle from fat, so a lean, heavily trained athlete can land in the overweight or obese band while carrying very little fat. It also ignores where fat is stored, and abdominal fat carries more metabolic risk than the same mass on hips and thighs. It is not validated for pregnancy, and for people of South Asian descent several health bodies use lower thresholds because risk rises at a lower BMI.
Better companions than a replacement
Waist circumference and waist-to-height ratio catch what BMI misses. A waist measurement under half your height is a common rule of thumb. Body fat percentage adds more still. Use BMI as a first look, not a verdict — our guide on what BMI really tells you goes further, and the body fat calculator estimates the composition BMI cannot see.
Children and teenagers
BMI for anyone under 20 is read against age-and-sex percentile charts rather than fixed cut-offs, because body composition changes rapidly through growth. This calculator uses the adult thresholds only.