BMI Calculator

Body mass index is deliberately simple: weight in kilograms divided by height in metres squared. It was devised in the nineteenth century by the Belgian statistician Adolphe Quetelet as a way of describing populations, and it survives today because it needs only two numbers that any clinic can measure in seconds. For a large group of adults, average BMI correlates reasonably well with average body fat, which makes it a cheap and reproducible screening signal for public health work.

The trouble starts when a population statistic is applied to one person. BMI has no way of knowing what your weight is made of. Muscle is considerably denser than fat, so a trained rower or rugby player can register as "obese" on the scale while carrying very little body fat. In the opposite direction, someone with low muscle mass and significant abdominal fat can sit comfortably in the "normal" band while carrying real metabolic risk — a pattern sometimes called normal-weight obesity. The index also says nothing about where fat sits, and visceral fat around the organs is far more strongly associated with cardiovascular and diabetes risk than fat on the hips and thighs.

There are groups for whom the adult thresholds simply do not apply. Children and teenagers are assessed against age-and-sex percentile charts rather than fixed cut-offs, because a growing body's expected BMI changes year on year. Pregnancy invalidates the measure entirely. Older adults lose height through spinal compression, which inflates BMI without any change in body composition. And several health bodies use lower overweight and obesity thresholds for people of South Asian descent, because cardiometabolic risk appears at a lower index in those populations. Treat the number here as one input among several — alongside waist circumference, blood pressure, blood work, activity level, and a conversation with a clinician who knows your history.