BMIBody mass index

Body mass index is weight in kilograms divided by height in metres squared. It was devised in the nineteenth century as a way of describing populations, and it does that job reasonably well.

The WHO categories are the familiar ones: under 18.5 underweight, 18.5 to 24.9 healthy weight, 25.0 to 29.9 overweight, and 30.0 and above obesity, subdivided into classes. Different thresholds apply for some Asian populations, where cardiometabolic risk rises at lower values.

Its central limitation is that it cannot see composition. Weight is weight, whether muscle or fat, so a muscular athlete is routinely classified as overweight and a sedentary person with little muscle and substantial abdominal fat can sit comfortably in the healthy range.

It also cannot see distribution, which is the more important omission. Fat around the organs carries far more metabolic risk than fat elsewhere, and BMI treats both identically. That is why waist to height ratio, with 0.5 proposed as a global boundary in a review of 78 studies, predicts cardiometabolic outcomes more often than BMI does.

None of that makes it useless. At population level it tracks health outcomes well, it costs nothing, it needs no equipment, and for most people who are not very muscular it gives a reasonable first indication.

The honest framing is that it is a screening tool rather than a diagnosis, and that a result near a threshold means look more closely rather than act.

In practice, use our BMI calculator for the number, then the waist to height ratio calculator for the one that carries more information, and treat neither as a verdict on a person.

Sources

• World Health Organization. Obesity and overweight fact sheet (adult BMI categories)
• Browning LM, Hsieh SD, Ashwell M. A systematic review of waist-to-height ratio as a screening tool for the prediction of cardiovascular disease and diabetes: 0.5 could be a suitable global boundary value. Nutr Res Rev 2010;23:247-269