BMI calculator
Body mass index from height and weight, and the categories the WHO defines.
BMI
24.5
Healthy weight. WHO category: 18.5 to 24.9.
What this does not capture
- BMI cannot distinguish muscle from fat. A muscular person and a sedentary person of the same height and weight get the same number.
- It says nothing about where fat is carried, and abdominal fat carries the cardiometabolic risk that whole-body weight does not.
- The category thresholds were derived largely in European-ancestry populations. WHO notes lower cut-points are appropriate for some Asian populations, and several countries use them.
- It is a population screening measure. It was never designed to describe an individual, and a 2025 Lancet Commission set out clinical obesity criteria that use BMI only as a first step.
Where the formula comes from
The ratio was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was describing the distribution of body sizes across a population rather than assessing anyone's health. It acquired the name body mass index in 1972 and entered clinical use because it is cheap, needs only a scale and a tape measure, and correlates reasonably well with body fat across large groups.
That last point is the whole case for it, and also its limit: a measure that behaves well across a population can behave badly for the individual standing in front of you.
What the categories mean
The WHO thresholds — under 18.5, 18.5 to 24.9, 25.0 to 29.9, and 30.0 and above — mark where population-level risk of several conditions begins to rise. They are not a diagnosis, and a number a fraction above or below a line does not describe a meaningful change in anyone's health.
In 2025 a Lancet Diabetes & Endocrinology Commission proposed separating clinical obesity, where excess adiposity is already causing organ dysfunction, from preclinical obesity, where it is not. Under that framework BMI is a screening step rather than the finding itself.
Sources
- Obesity and overweight fact sheet — World Health Organization
- Definition and diagnostic criteria of clinical obesity — The Lancet Diabetes & Endocrinology Commission