BMI calculator

Body mass index divides your weight in kilograms by the square of your height in metres. That is the whole formula. It was designed in the 1830s to describe populations, not individuals, and it has been doing a job it was never built for ever since.

What it gives you is a single number that correlates loosely with body fat across large groups. What it cannot do is tell those groups apart. It has no way of knowing whether a kilogram is muscle, fat, bone, or the large lunch you ate an hour ago.

These cut-offs are the conventional adult categories. They are round numbers chosen for convenience, and a BMI of 24.9 is not meaningfully different from 25.1 despite sitting on opposite sides of a line.

A muscular person is the classic case. Muscle is denser than fat, so someone who lifts seriously can land in the overweight band while carrying very little fat. The BMI is not wrong about their mass. It is wrong about what that mass is.

It also drifts with height. The formula squares height while bodies scale closer to a cube, so tall people read slightly high and short people slightly low for the same build. And it says nothing at all about where fat sits, which is the part most associated with health risk.

If you want a second reading that accounts for shape, waist circumference is more informative and takes ten seconds with a tape measure.

The honest use of BMI is as one rough input among several, and as a trend rather than a verdict. A number that moves steadily over six months tells you something. The same number read once tells you almost nothing.

If you are tracking a body-related habit, the weekly direction of travel is the useful signal. Daily weight swings of a kilogram or two are mostly water and food in transit.