What BMI does and does not tell you
Where the Body Mass Index came from, how the WHO bands work, and the cases where BMI misreads a healthy body.
A 19th century formula, still in use
BMI began as the Quetelet Index, devised in the 1830s by the Belgian mathematician Adolphe Quetelet to describe the average build of a population. It was never designed to judge an individual. The name Body Mass Index arrived in the 1970s, when researchers looked for a simple weight-for-height ratio that correlated with body fat across large groups. Its staying power comes from its simplicity: you need only a scale and a tape measure, and the arithmetic fits on one line.
How the WHO bands were set
The World Health Organization anchors its four bands at 18.5, 25 and 30. These thresholds were chosen because, across large studies, the risk of weight-related illness starts to climb noticeably above 25 and more steeply above 30, while very low values below 18.5 carry their own risks. The bands are the same for adult men and women, which is part of why they are a blunt instrument at the level of one person, whose ideal weight depends on frame and composition.
Where BMI misreads a body
Because BMI uses only mass and height, it cannot see what the mass is made of. A muscular athlete and a sedentary person of the same height and weight get the same BMI even though their body fat differs sharply. Older adults who have lost muscle can show a healthy BMI while carrying excess fat, and very tall or very short people are slightly penalised or flattered by the height-squared term. These are systematic gaps, not random error.
Better questions to ask alongside BMI
BMI is most useful as one line in a fuller picture. Waist circumference and the waist-to-height ratio capture central fat, which drives much of the metabolic risk that BMI only hints at. Body fat percentage from a scan or calipers separates muscle from fat directly. Blood pressure, resting heart rate, blood sugar and cholesterol describe function rather than shape. If your BMI surprises you, these measures usually explain why.