Where BMI Came From, and Why 25 Became the Line

Two measurements go in and one number comes out, and that arithmetic has not changed in nearly two centuries. What has changed, more than once, is where the boundaries between the categories sit and who is expected to do something about them. The history is worth having before you read your own figure as a verdict.

Before any of it: this is general information assembled by a software team. Nobody with medical training has reviewed this page, none of it is medical advice, and none of it can tell you whether your particular result means anything.

An index built to describe crowds

Adolphe Quetelet was a Belgian astronomer who spent much of his working life applying observational statistics to human beings. The ratio of weight to the square of height appears in his 1835 treatise on what he called l'homme moyen, the average man, as a way of describing how build is distributed across a population.

He was not proposing a screening test. There was no clinical data behind the ratio, no cohort followed for outcomes, and no suggestion that a single person's value carried a meaning. It was a descriptive statistic for groups, which is a different kind of object entirely.

The modern name arrived 137 years later. Ancel Keys and four co-authors, writing in the Journal of Chronic Diseases in 1972, compared several simple height-and-weight ratios against measured body fat in cohorts drawn from five countries. Weight over height squared tracked measured fatness better than the alternatives and depended least on how tall the subject was, so they recommended it and gave it the name body mass index. The same paper noted that the index suited population studies rather than individual diagnosis. That qualification did not travel with the formula.

Why the height is squared

The exponent is an empirical fit rather than a law. If people scaled like geometrically similar solids, mass would rise with the cube of height and the sensible denominator would be height cubed. Real human beings do not scale that way — they get proportionally broader as they get taller — and Quetelet's measurements put adult weight nearer the square.

The square fits the middle of the distribution well and drifts at the ends, in a direction that is systematic rather than random. Nick Trefethen, a numerical analyst at Oxford, argued in a 2013 letter to The Economist that weight scales closer to height to the power 2.5, and proposed 1.3 times weight divided by height to the 2.5 as a replacement.

Run two people who share a BMI of exactly 25.0 through both. Someone 1.60 m tall weighing 64 kg comes out at 25.7 on the alternative index; someone 1.90 m weighing 90.25 kg comes out at 23.6. Two points of separation between people the standard index calls identical, purely from height. The proposal has not been adopted, and the reason is not that it is wrong — it is that most of an index's value comes from everyone using the same one.

The week that millions of people changed category

Through the 1980s and early 1990s, United States federal guidance placed the overweight boundary at roughly 27.8 for men and 27.3 for women, values taken from the 85th percentile of a national survey rather than from any biological threshold. The National Heart, Lung, and Blood Institute's clinical guidelines, released in 1998, replaced both figures with a single cut-off of 25 applied to everyone, aligning United States practice with the classification the World Health Organization had adopted.

Nobody's body changed. Tens of millions of American adults were reclassified overnight by an edit to a document. Whatever risk each of those people carried on the Monday, they carried the identical risk on the Friday.

That is what a cut-off is: an administrative line ruled through a smooth, continuous distribution because guidance has to say something and continuous variables are awkward to write policy about. Someone at 24.9 and someone at 25.1 are, physiologically, the same person twice. Several health authorities also apply lower action points to South and East Asian populations, which is another way of admitting that a single global boundary was always a compromise.

The mortality evidence is messier than the table suggests

The bands are usually presented as a ladder of increasing danger. The largest attempt to check that against death records complicated it considerably.

Katherine Flegal and colleagues pooled 97 studies covering roughly 2.88 million people and published the result in JAMA in 2013. Grade 1 obesity, 30 to 35, showed no statistically significant excess of all-cause mortality. The overweight band, 25 to 30, was associated with mortality slightly below the healthy-weight band. Only grades 2 and 3, above 35, showed the clear excess the ladder predicts.

The paper was attacked hard and the criticisms are serious ones. Smokers tend to be lighter and die earlier, which drags the reference group down. People already ill often lose weight before they die, so illness can look like a consequence of thinness rather than a cause of it. The debate is genuinely unsettled, and anyone presenting either side of it as established fact is overselling. What it does establish is that the neat progression printed in the table is not something the mortality data simply confirms.

In 2023 the American Medical Association adopted policy explicitly acknowledging the index's limitations and recommending it be used alongside other measures rather than on its own.

What to do with your own number

Our BMI calculator prints the same standard bands, and it inherits every problem described above. It reports a population statistic. It does not know your muscle mass, your waist circumference, your blood pressure, your blood glucose, your family history or your age, and those are the things that carry the clinical information.

Treated as one input among several it is a reasonable one — cheap, reproducible, and comparable across time if you weigh yourself the same way. Treated as a diagnosis it is being asked to do a job it was never built for and that its own author declined to claim for it.

If you want a measurement that responds to shape rather than mass, a tape measure and the body fat calculator will get closer. If you want to know what your result means for you, that conversation belongs with a clinician who can see the rest of the picture.