Understanding BMI: What It Measures and What It Misses

Updated 2026-08-01 · 5 min read

BMI is one of the most widely used and most widely misunderstood health numbers. It is genuinely useful for the job it was designed to do, and genuinely misleading when stretched beyond it. The distinction is worth understanding before reading anything into your own number.

What it is

Body mass index is weight in kilograms divided by height in metres squared. That is the entire calculation. It uses no other information: not age, not sex, not body composition, not medical history.

The formula dates to the 1830s, devised by Belgian statistician Adolphe Quetelet, who was studying the characteristics of populations. It was never intended as a measure of an individual's health, and Quetelet said as much.

Where the categories come from

The familiar bands, published by the World Health Organization for adults, are: below 18.5 underweight, 18.5 to 24.9 healthy weight, 25.0 to 29.9 overweight, and 30.0 and above obese.

These thresholds were derived from statistical associations across large populations, mostly of European descent. They describe where health risks tend to rise on average across many thousands of people. They do not describe what is happening in any particular body, and the boundaries are conventions rather than sharp biological transitions. Nothing meaningful changes between a BMI of 24.9 and 25.0.

What BMI cannot see

It cannot distinguish muscle from fat. Muscle is denser than fat, so a very muscular person can register as overweight or obese while carrying little body fat. This is well documented among athletes, and it is the most frequently cited limitation.

It cannot see where fat is stored. Research consistently finds that fat around the abdomen carries different health implications than fat on the hips and thighs. Two people with identical BMI can have meaningfully different risk profiles. This is why waist circumference is often measured alongside BMI.

It says nothing about fitness or metabolic health. Blood pressure, blood sugar, cholesterol, and cardiorespiratory fitness are all measurable, all relevant, and all invisible to BMI.

Groups where it is especially unreliable

Children and adolescents. Adult categories do not apply at all. Growing bodies are assessed against age- and sex-specific percentile charts.

Older adults. Muscle mass declines and height is often lost with age, both of which distort the number.

Pregnant people. BMI is not a meaningful measure during pregnancy.

Athletes and physically heavy workers. High muscle mass reliably pushes BMI upward.

Different ethnic groups. Because the thresholds were derived largely from European populations, several health bodies recommend lower cut-offs for people of South Asian, South-East Asian, and East Asian descent, where elevated risk appears at lower BMI values.

So is it useless?

No, and dismissing it entirely overcorrects. BMI is cheap, requires only a scale and a tape measure, is consistent between clinicians, and works well for its actual purpose: comparing populations and flagging individuals who might benefit from a closer look.

The problem is not the measure. It is treating a rough screening tool as a verdict.

What "ideal weight" formulas are

Formulas such as Devine, Robinson, and Hamwi calculate a weight from height and sex. It is worth knowing where they came from: the Devine formula was published in 1974 to help calculate medication dosages, not to advise people on body weight.

They are simple linear formulas, they disagree with each other, and none reflects your build, frame, or body composition. Read them as rough reference points, not targets.

A reasonable way to use the number

Treat BMI as one crude data point. If it falls well outside the healthy band, that is worth a conversation with a doctor who can look at the fuller picture. If it sits near a boundary, the boundary itself is not precise enough to act on.

No calculator can assess your health. A clinician who can consider your history, measurements, blood work, and circumstances is a far better guide than any formula, including this one.

This article is general information, not medical advice. For guidance about your own health, speak to a qualified healthcare professional who can consider your full history.