What Body Mass Index measures
Body Mass Index is a single number derived from your height and weight. Adolphe Quetelet, a 19th-century Belgian statistician, first proposed the ratio of weight in kilograms to the square of height in metres as a population-level indicator of average body size. The World Health Organization adopted the same formula in the 1990s as a screening tool that flags individuals who may be underweight, overweight, or at risk of obesity-related conditions.
BMI is intentionally simple. It uses only two inputs, can be calculated mentally with a bit of practice, and applies the same formula worldwide. That simplicity is also its weakness: BMI does not distinguish muscle from fat, does not account for bone density, frame size, or fat distribution, and does not reflect cardiovascular fitness.
WHO weight categories for adults
- Below 18.5 — underweight.
- 18.5 to 24.9 — normal range.
- 25.0 to 29.9 — overweight.
- 30.0 to 34.9 — obesity class I.
- 35.0 to 39.9 — obesity class II.
- 40.0 and above — obesity class III (severe).
How to read your result
Treat BMI as a starting point, not a diagnosis. An elite athlete with substantial muscle mass can register as overweight; an older adult with reduced muscle can register as normal while carrying excess body fat. For a more complete picture, combine BMI with waist circumference, body fat percentage, blood pressure, and lipid panel readings, and discuss the trend with a qualified medical professional. The tool does not store, log, or transmit the numbers you enter.