BMI Calculator

📖 What BMI Does and Does Not Tell You

Body Mass Index was devised in the 1830s by Adolphe Quetelet, a Belgian astronomer and statistician, as part of a project to define the statistical average of human characteristics. He was not studying health - he was studying populations, and he needed a simple way to compare body sizes. The formula (weight divided by height squared) was chosen for mathematical convenience, not medical accuracy.

The index was largely forgotten until the 1970s, when physiologist Ancel Keys popularised it as a screening tool for population-level health studies. Even Keys acknowledged that BMI was designed for populations, not individuals. It measures exactly one thing: the ratio of your weight to the square of your height. It cannot distinguish fat from muscle, and it tells you nothing about where your body stores that fat - which matters more than total fat for metabolic health.

BMI remains in use because it is quick, free, and non-invasive. At a population level, it correlates reasonably well with health outcomes. For an individual, it is a starting point - a signal worth investigating, not a verdict. If your BMI suggests a concern, or if you have reason to believe it does not reflect your actual body composition, a waist-to-height ratio or a body composition assessment from a qualified professional gives a more complete picture.

For a deeper look at the history, limitations, and alternatives, read our guide on what body composition numbers really tell you.