BMI calculator (and its limits)

What BMI actually measures

Body mass index is your weight in kilograms divided by your height in metres squared. It was devised in the 1830s by Adolphe Quetelet, a statistician, as a way of describing populations — not individuals. That origin matters, because it explains both what BMI is good at and where it fails.

Where BMI misleads

BMI cannot distinguish muscle from fat. A lean, well-trained rugby player will often register as “overweight” or even “obese” despite carrying very little body fat, because muscle is denser than fat. The reverse also happens: someone sedentary with low muscle mass can sit inside the healthy range while carrying an unhealthy amount of fat — sometimes called “normal weight obesity”.

It also ignores fat distribution, which matters clinically. Visceral fat stored around the organs carries considerably more health risk than the same weight of fat carried on the hips and thighs, and BMI treats them identically. The standard cut-offs were derived largely from European populations, and health authorities recognise that appropriate thresholds differ for people of South Asian and some Pacific descent.

Better numbers to track

Waist circumference is a more informative single measurement, because it reflects abdominal fat directly — the pattern most strongly associated with metabolic risk. Waist-to-height ratio is better still, and the rule of thumb is simple: keep your waist under half your height. Beyond that, how your training performance, energy, sleep and clothes are trending will tell you more about your progress than any single index.

Use BMI as one rough screening number among several, not as a verdict. If it places you outside the healthy range and you are unsure what that means for you specifically, that is a conversation for your GP rather than a calculator.