BMI: what it measures, and four things it cannot tell you
BMI is a useful population screening tool and a blunt individual one. Here is what the number means, where it misleads, and what to look at alongside it.
Body Mass Index is your weight in kilograms divided by your height in metres squared. That is the whole calculation. It was devised in the 1830s by a statistician studying populations, not by a physician assessing patients — and that origin explains both its usefulness and its limits.
What the number is actually doing
Dividing by height squared is a normalisation trick. Heavier people are usually taller, so raw weight tells you little on its own. Scaling by height² produces a figure that is roughly comparable between a 155 cm adult and a 190 cm one, which is what makes population statistics possible.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal range |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
Four things it cannot see
It cannot tell muscle from fat. Muscle is denser than fat, so athletes routinely register as overweight. A lean, heavily trained rugby player and a sedentary person of the same height and weight get identical BMIs and very different health risks.
It cannot see where fat sits. Fat around the abdomen carries more metabolic risk than fat on hips and thighs. Two people at BMI 27 can face quite different risk profiles depending on distribution — which is why waist-to-hip ratio adds information BMI structurally cannot.
It does not adjust for age. Body composition shifts over decades; an older adult can lose muscle and gain fat while their BMI stays flat.
It was calibrated on European populations. Risk thresholds differ by ancestry — several health bodies use a lower overweight cutoff of 23 for people of South Asian descent, because metabolic risk appears at lower BMIs.
What to look at alongside it
- Waist circumference, or waist-to-hip ratio, for fat distribution.
- Body fat percentage, if you have a reasonable way to estimate it.
- The direction of travel — a BMI trending up over two years says more than a single reading.
- How you actually feel: energy, sleep, and what you can do physically.
If your BMI sits outside the normal range, the useful next step is a conversation with a clinician who can weigh it against everything the formula cannot see.
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