BMI Calculator

About the BMI Calculator

The BMI Calculator computes your Body Mass Index from your height and weight, shows your healthy weight range, and tells you which BMI category you fall into — a quick screening tool for weight relative to height.

What BMI is, and what it was designed for

BMI is your weight in kilograms divided by the square of your height in metres. It was devised in the 1830s by a statistician studying populations, not patients, and that origin explains most of its limitations. It is a cheap, fast screening number that works reasonably well across large groups and considerably less well on any particular individual.

That's not a reason to ignore it. It's a reason to treat it as one line in a picture that also includes waist measurement, blood pressure, blood work, activity, sleep and family history — none of which a height and a weight can see.

Where it misleads

• Muscle reads as fat. The formula cannot distinguish them, so it routinely classifies muscular people as overweight or obese. A rugby player and a sedentary person of the same height and weight get an identical score.
• It ignores where fat sits, which is the part that actually correlates with metabolic risk. Waist-to-height ratio — keeping your waist under half your height — is a better single number for that, and it needs nothing but a tape measure.
• The thresholds don't fit everyone. They were derived largely from European populations, and several health authorities use lower cut-offs for people of South and East Asian descent, where risk rises at a lower BMI.
• It isn't valid for children, during pregnancy, or for adults over about 65, where a slightly higher BMI is associated with better outcomes, not worse.
• The category boundaries are lines on a continuum. There is no meaningful health difference between 24.9 and 25.1, and treating one as a pass and the other as a failure is a misreading of what the number is.

Reading your result usefully

The healthy weight range shown is simply the equation solved backwards — the weights that would put you at BMI 18.5 and 24.9 at your height. The "ideal weight" figure is the weight at BMI 22, near the middle of that band. Both are reference points, not targets handed down from anywhere.

The most useful thing you can do with a BMI is watch it over time rather than judge it once. A number that has moved three points in a year says something worth paying attention to; the same number in isolation says very little. And if it sits well outside the middle band, the productive next step is a conversation with a doctor who can order the tests BMI is standing in for — not a diet chosen from a search result.

Works well with

Sleep and weight are the two things people most often try to change in the same week, and they're more connected than they look — short sleep affects appetite regulation directly. The Sleep Calculator is the usual companion here. The Age Calculator is next door if you want exact age for a health form. The formula, the source of the category boundaries, and the full list of caveats are set out on the methodology page.

Frequently Asked Questions