BMI Calculator

What BMI measures, and what it was designed for

The index was devised in the 1830s by Adolphe Quetelet, a statistician studying the characteristics of populations — not a physician assessing individuals. That origin explains both its strengths and its limits. As a way of comparing groups it works well: it needs only two cheap measurements, it correlates reasonably with body fat across large samples, and it makes decades of health data comparable. As a statement about one person it has an obvious flaw, which is that it cannot tell what the weight is made of. Muscle is denser than fat, so athletes routinely score as overweight, and someone with low muscle mass can sit inside the healthy band while carrying a risky amount of fat. It also ignores where fat is distributed, which matters: abdominal fat carries more cardiometabolic risk than the same mass elsewhere, and waist measurement captures that where BMI does not.

Reading the healthy range rather than the number

For most people the more useful output is not the index but the weight range that corresponds to a healthy one for their height, which is what inverting the formula gives. It converts an abstract figure into something actionable — a range in the units you actually weigh yourself in. It is worth noticing how wide that range is: at 178 cm it spans roughly 59 to 79 kilograms, a twenty-kilogram band all classified as healthy. That width is a reminder that BMI is a coarse screen with broad boundaries, not a target to optimise toward. Movement within the band is not meaningful, and treating a single number as a goal is the most common way this measure gets misused.