What Your BMI Does and Doesn't Tell You

Body mass index is the most widely used health number in the world, and one of the most widely misunderstood. It appears on medical records, insurance forms and fitness apps as though it were a measurement of your body. It is not. It is a ratio, invented for a purpose quite different from the one it now serves.

Where BMI came from

In the 1830s, a Belgian astronomer and statistician named Adolphe Quetelet was looking for mathematical regularities in human populations. He found that across a population, weight scales roughly with the square of height, and defined what he called the Quetelet Index to describe it.

Quetelet was explicit that the index described populations, not individuals. He was not a physician and had no interest in diagnosing anyone.

It acquired its modern name in 1972, when the physiologist Ancel Keys tested several weight-for-height ratios and found Quetelet's was the best available proxy for body fat in large studies. "Best available" is doing considerable work in that sentence — Keys also noted it was inappropriate for individual diagnosis. Insurers and health systems adopted it anyway, because it needs only a tape measure and a set of scales.

What it is genuinely good at

Three things, and it is genuinely good at all of them.

Population screening. Across thousands of people, average BMI tracks average body fat closely enough to be useful for public health monitoring.

Cheapness. Two measurements anyone can take, with no equipment beyond scales and a wall. No other body composition measure comes close on cost and availability.

Tracking change in one person. If your height is stable, a BMI moving from 31 to 27 over a year reflects a real change in your body. The direction and size of movement are more informative than the absolute number.

Where it fails

It cannot distinguish muscle from fat. Muscle is roughly 18% denser than fat. A muscular athlete routinely lands in the "overweight" or "obese" range while carrying very little body fat. This is the failure everyone knows about, and it is real — though it affects fewer people than gym folklore suggests.

It ignores where fat sits. Visceral fat around the organs carries substantially more metabolic risk than subcutaneous fat on the hips and thighs. Two people with identical BMIs can have very different risk profiles. Waist-to-height ratio — keep your waist under half your height — captures this better and is just as easy to measure.

It was calibrated on European populations. Risk of type 2 diabetes and cardiovascular disease rises at lower BMIs in South Asian, Chinese and other Asian populations. The WHO and several national bodies recommend lower thresholds, sometimes treating 23 rather than 25 as the overweight boundary.

It drifts with age. Adults lose muscle and bone density from roughly the fourth decade onward. The same BMI at 70 typically represents more body fat than at 30. Some evidence suggests a slightly higher BMI is protective in older adults.

It does not apply to children, or during pregnancy. Children are assessed against age- and sex-specific percentile charts because body composition changes rapidly during growth. Pregnancy makes the ratio meaningless.

What to use alongside it

You do not need to abandon BMI. You need to stop treating it as the whole answer.

• Waist circumference, or waist-to-height ratio, adds the fat-distribution information BMI lacks, at no extra cost.
• Blood pressure, fasting glucose and lipid panel measure the things weight is a proxy for. If those are healthy, a BMI of 27 means something quite different than if they are not.
• What you can actually do — climb stairs, carry shopping, sleep well — is information no ratio captures.

The sensible way to read your number

Treat BMI as a prompt, not a verdict. A number outside the healthy range is a reason to look at the other measures above and, if several point the same way, to talk to a doctor. A number inside the healthy range is not proof that everything is fine, and a number outside it is not proof that anything is wrong.

You can work out your own with the BMI calculator, which also shows the healthy weight range for your height and the full category chart.