How it is calculated
Divide weight in kilograms by height in metres, squared. Someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. The BMI Calculator does the same sum and adds the category.
The adult categories
The World Health Organization groups adult BMI like this: under 18.5 is underweight, 18.5 to 24.9 is normal, 25 to 29.9 is overweight, and 30 or more is obese. The bands are a screening aid. Falling in the “normal” range does not mean a person is healthy, and falling outside it does not mean they are not.
What BMI cannot tell you
BMI uses only height and weight, so it cannot tell muscle from fat. A strength athlete can land in the overweight band with very little body fat, and an older adult who has lost muscle can look “normal” while carrying more fat than is healthy. It also says nothing about where fat sits on the body, and that matters for health risk.
The standard cut-offs were built mainly from European populations. A WHO expert consultation reported in The Lancet in 2004 (Nishida et al., 363:157–163) that many Asian people already have a high risk of type 2 diabetes and heart disease at BMIs below 25. It kept the international categories but added public health action points at 23.0 and 27.5. If that applies to you, ask your doctor which thresholds they use.
Children and teenagers
The adult bands do not apply to people under 20. For children and teens, BMI is compared with growth charts for the same age and sex and read as a percentile; the CDC explains this in its child and teen BMI guidance. A paediatrician is the right person to interpret the result.
Where the number comes from
The formula goes back to the Belgian mathematician Adolphe Quetelet in the 1830s and 1840s, who looked for a way to describe the “average man” across a population. The name “body mass index” was only given to it in 1972 by the physiologist Ancel Keys. It was designed for studying groups, which is why it is a better tool for population statistics than for judging one person.
Sources: WHO fact sheet on obesity and overweight; WHO Expert Consultation, Lancet 2004;363:157–163; CDC child and teen BMI. General information only, not medical advice.