BMIBody mass index
Weight divided by height squared — a crude population screening measure, not a diagnosis.
Body mass index is mass in kilograms over height in metres squared. It was devised in the nineteenth century as a population statistic, and the familiar categories — under 18.5, 18.5 to 25, 25 to 30, above 30 — are conventions layered on afterwards.
It matters because at population scale it does predict health outcomes reasonably well and costs nothing to collect. For any individual it is much weaker: it cannot distinguish muscle from fat, ignores where fat sits, and its risk thresholds differ meaningfully by ethnicity, with lower cut-offs recommended for South Asian populations.
In practice it is best treated as a first screen rather than a verdict. Waist-to-height ratio is a better single number for metabolic risk, and any body-fat estimate tells you more about composition. A muscular person flagged as overweight by BMI is the classic false positive.
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Written and reviewed by Mathew Beale, MSc Biotechnology, University of Reading.
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