BMI calculator

How to use it

1. Choose metric (cm / kg) or imperial (ft, in / lb).
2. Enter your height and weight — the BMI updates as you type.
3. The category shown is the WHO adult classification.

A nineteenth-century population statistic applied to one person

Adolphe Quetelet devised the ratio in the 1830s as a statistician describing the distribution of a population, not as a clinician assessing a patient. The name body mass index came from Ancel Keys in 1972, who judged it the best of the simple weight-for-height indices for epidemiological work while stating plainly that it was not suited to individual diagnosis. That caveat has not travelled as well as the number.

The formula is weight in kilograms divided by height in metres squared, and the WHO adult bands are below 18.5, 18.5 to 24.9, 25.0 to 29.9, and 30.0 and above, subdivided again at 35.0 and 40.0. The squared term is a known artefact: mass scales roughly with the cube of height, so the index tends to read high for tall people and low for short ones regardless of body composition.

Why several health bodies use lower thresholds for some populations

A WHO expert consultation in 2004 concluded that cardiometabolic risk begins to rise at lower BMI values in Asian populations than the standard cut-offs suggest. NICE consequently recommends lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, using 23.0 as the increased-risk point and 27.5 where the standard figure would be 30.0.

The index also cannot distinguish muscle from fat or indicate where fat is carried, which is why a trained athlete routinely reads as overweight, and why it is normally paired with other measures such as waist-to-height ratio. A figure from this page is a screening statistic, not a diagnosis and not medical advice. Interpreting what it means for one particular body is a conversation for a clinician who can see the rest of the picture.

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