What BMI really tells you — and the four things it cannot see

Body Mass Index is the most widely used health number in the world and one of the most widely misunderstood. It is neither the meaningless relic its critics describe nor the diagnosis its defenders sometimes imply. It is a cheap, fast screening ratio with a specific design purpose — and knowing that purpose is what lets you read it correctly.

Where the formula came from

The ratio was devised in the 1830s by the Belgian mathematician and statistician Adolphe Quetelet, who was studying the distribution of human characteristics across populations. He was not trying to assess individual patients; he wanted a measure of build that stayed roughly constant across heights so that populations could be compared. It was named Body Mass Index in 1972 by Ancel Keys, who tested several weight-height ratios and found this one correlated best with body fat in large samples.

The standard ranges

These cut-offs are statistical thresholds where population-level health risk begins to rise, not cliffs where health changes. A BMI of 24.9 and a BMI of 25.1 describe practically identical bodies. Treat the boundary as a soft signal, not a verdict.

Four things BMI cannot see

1. The difference between muscle and fat

BMI uses total mass. Muscle is denser than fat, so a trained athlete can carry a BMI of 28 with 10% body fat and be classified as overweight by a measure that has no way of noticing. This is the best-known limitation and the reason BMI should never be the only number in an athletic context.

2. Where the fat is

Visceral fat around the abdomen carries substantially higher metabolic risk than subcutaneous fat on hips and thighs. Two people with the same BMI and the same body fat percentage can have very different risk profiles depending on distribution — which is why waist circumference adds real information that BMI structurally cannot provide.

3. Age and body composition changes

Adults lose muscle mass gradually from around the fourth decade. Someone whose weight has not changed in twenty years may have replaced several kilos of muscle with fat while their BMI stayed identical. The number is stable; the body underneath it is not.

4. Population differences

The standard thresholds were derived largely from European-descended populations. The WHO notes that people of Asian descent tend to show elevated cardiometabolic risk at lower BMI values, and several countries use lowered action points — commonly 23 for overweight and 27.5 for obesity — for these populations.

What to measure alongside it

• Waist circumference — measured at the navel, standing, after a normal exhale. Risk rises above about 94 cm for men and 80 cm for women, with higher thresholds signalling substantially higher risk.
• Waist-to-height ratio — keep your waist under half your height. It is a single number, needs no table, and predicts cardiometabolic risk at least as well as BMI.
• Body fat percentage — from skinfolds or bioimpedance. Less precise than laboratory methods but far more informative than mass alone.
• Trend over time — one measurement is a snapshot; the direction of travel over months is the actual signal.

Where BMI is still the right tool

For large-scale public health work it is unmatched: it needs only a scale and a tape measure, it is free, it is reproducible, and decades of epidemiological data are indexed to it. For an individual it is a reasonable first screen — a fast way to ask whether a longer conversation about weight is warranted. What it is not is a diagnosis, a fitness score or a measure of anyone's worth.

Read it as one line in a report rather than the headline, and it does exactly the job it was built to do.