Body mass index, or BMI, is a single number that relates your weight to your height. Doctors and public health agencies use it as a quick screening tool. This article explains the formula, the official categories, how to work it out by hand and what the number cannot tell you. It is general information, not medical advice.

What BMI actually is

BMI stands for body mass index. It was designed to describe, for groups of adults, how weight compares with height. It is not a measurement of body fat and it does not look inside the body. Two people with the same height and weight get exactly the same BMI, even if one of them is very muscular and the other is not.

That is why health professionals call it a screening measure. It is cheap, fast and needs only a scale and a tape measure, so it is a handy first look. A high or low result is a reason to ask questions, not a diagnosis.

The formula

In metric units the formula is weight in kilograms divided by height in metres squared: BMI = kg / m2.…