Body mass index divides weight by the square of height. It was devised in the 1830s by Adolphe Quetelet as a statistical tool for describing populations, not for assessing individuals — a distinction that has been widely lost.
The standard bands
| BMI | Label used by WHO |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal range |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
What it cannot see
BMI knows only two numbers, so it cannot distinguish muscle from fat, cannot tell where fat is distributed, and takes no account of age, sex, frame size or ancestry. Athletes with high muscle mass routinely score as overweight. Older adults who have lost muscle can score as normal while carrying substantial fat.
The squared height term also biases the scale: it tends to overestimate for tall people and underestimate for short ones, because human bodies do not scale as neat squares.
Better individual measures
Waist circumference and waist-to-height ratio track abdominal fat, which is the deposit most strongly linked to metabolic risk, and both are almost as easy to measure. Clinically, BMI is used as a first filter and then followed up with something more informative. A single number from two measurements cannot tell you whether you are healthy; a doctor with your full picture can.