BMI (body mass index)
The ratio of weight to height squared, used to classify body size across populations, but whose interpretation for an individual calls for a great deal of caution.
BMI is calculated by dividing weight in kilograms by the square of height in metres. The WHO uses simple thresholds: under 18.5 for underweight, 18.5 to 25 for a healthy weight, 25 to 30 for overweight, and above 30 for obesity.
Its strength is its simplicity: two measurements are enough, which makes it a very effective epidemiological tool for comparing populations and following trends. Our BMI calculator applies these thresholds.
Its major limitation is that it does not tell muscle from fat. A muscular athlete can have a BMI of 28 without any excess body fat, while a sedentary person with a normal BMI may have high adiposity and a real metabolic risk. The index also ignores fat distribution, even though this is what weighs most on cardiovascular risk.
It is also less relevant in older people, whose body composition changes, in children (for whom BMI-for-age growth charts are used) and during pregnancy. Waist circumference provides complementary information that is often more telling, as it reflects abdominal fat. For an individual, BMI is a starting point, never a diagnosis.