What is BMI (body mass index)?

In one line

BMI (body mass index) is weight divided by height squared, a number that shows how heavy you are for your height.

What is BMI (body mass index)?
The number on every health checkup report
Look at a health checkup or body composition report and you will find a number called 'BMI'. Anyone can work it out with just height and weight. Here is what BMI tells you, and what it does not, step by step.
In one line
How heavy you are for your height
BMI is your weight divided by your height squared, and it shows how heavy you are for your height. In Korean it is called 'chejillyang jisu'. It is often used as a first step to judge whether someone carries too much body fat.
How to calculate
Divide weight (kg) by height (m) twice
Take weight in kg and height in m, and divide the weight by the height twice. With a height of 1.75 m and a weight of 70 kg, 70 ÷ 1.75 ÷ 1.75 gives about 22.9. The result is usually stated as a plain number without units.
Where it came from
A statistician's formula from about 200 years ago
This formula was put forward in 1832 by the Belgian statistician Quetelet. He found that the weight of fully grown people rises roughly with the square of their height. In 1972 the American scientist Keys compared several formulas, judged this one better, and named it 'BMI'.
World standard
Overweight from 25, obesity from 30
The World Health Organization classes an adult BMI of 25 or more as overweight and 30 or more as obesity. Under 18.5 is underweight, and in between is normal. Children who are still growing use separate standards matched to age and sex.
Korean standard
Pre-obesity from 23, obesity from 25
Korea follows Asian standards and draws the lines at lower numbers. The Korean Society for the Study of Obesity treats a BMI of 23 to 24.9 as pre-obesity and 25 or more as obesity. This is because, at the same BMI, Koreans develop obesity-related diseases more readily than Western populations.
Why use it
Easy to get, and linked to risk of death
BMI can be worked out right away for free from height and weight. When adults who never smoked and had no disease were followed for years, the risk of death was lowest at a BMI of 20 to 25. Risk rose below this range, and rose as BMI went higher above 25.
Weak point
It cannot tell fat from muscle
BMI cannot tell whether body weight is fat, muscle or bone. So BMI alone does not show how much fat is in the body. The US Centers for Disease Control and Prevention also says BMI should be looked at along with other checks such as blood pressure.
Common misconception
Does a BMI over 25 mean you are fat?
Not necessarily. With a lot of muscle, BMI comes out high even with little fat. When the body fat share (body fat percentage) of young people on sports teams who trained regularly was measured directly, it was common to find a BMI over 25 without much fat.
Summary
A number for how heavy you are for your height
BMI is weight divided by height squared, and it shows how heavy you are for your height. Because people with a lot of muscle get a high BMI even with little fat, body fat is checked separately with body fat percentage. Related terms: body fat percentage, body composition analysis, visceral fat.