Body mass index

The **body mass index** (or **BMI**) is a measure which shows whether people have the right weight for their height. The World Health Organisation (WHO), governments and health workers use it. It is also sometimes called the **Quetelet index**.

Many people use the body mass index as a rough indication: it is easy to calculate but it leaves out many factors that should also be taken into account. That way further tests are often performed if the BMI shows there is a problem.

## History

The BMI was invented by Adolphe Quetelet between 1830 and 1850.

A scientist named Ancel Keys first used the term _body mass index_ in 1972.  He wrote that governments should measure the BMI of adults to find out whether they are too fat or too thin.

## Calculation

To find somebody's BMI:
- measure their weight (body mass) in kilograms or pounds
- measure their height in meters or inches
- divide their weight by the square of their height
- if using pounds and inches, multiply the answer by 703
- **BMI calculator**
This is the formula:

For example, a man whose weight is  and height is  has a BMI of:
A man whose weight is  and height is  has a BMI of:

## Overweight or not

Health organisations, including the World Health Organisation (WHO), use the BMI to help decide whether people are too fat or too thin.  The WHO uses these numbers for adults:
|Category|BMI range|
| --- | --- |
|_Seriously Underweight_|less than 16.49 (--16.49)|
|_Underweight_|between 16.5 and 18.49 (16.5--18.49)|
|_Normal_|between 18.5 and 24.99 (18.5--24.99)|
|_Overweight_|between 25 and 29.99 (25–29.99)|
|_Obese_|30 or more (30--)|

Some people disagree with these numbers.  Some experts think the _Overweight_ range for people with Asian body types should be 23–27.49, not 25–29.99.  Some experts think the lowest _Normal_ BMI should be 20 for men, and 18 for women.
The old BMI definition of overweight was 27. This changed in 1998.

## Uses

BMI is useful for governments who want to know about the health of their people.  For example, in the United States in 2007, 74% of adults had a BMI above 25 (_Overweight_ or _Obese_) and 27% of adults had a BMI above 30 (_Obese_).  Many of these people will become ill because they are too fat.  This information helps the United States government decide what to do about the health of its people.

BMI is less useful for finding out whether one person is too fat or too thin.  BMI does not say how much weight is fat, how much is muscle and how much is bone.  For example, many athletes have a high BMI.  They are not fat.  Their BMI is high because they have lots of heavy muscle.  Other people have lots of fat and not much muscle.  They are too fat but their BMI is normal.

Even so, BMI is popular.  It is easy to use and easy to understand, and it gives useful information for people with "normal" bodies.

## Variations

The Ponderal Index is similar to the **body mass index**, but the height is cubed, not squared.  The formula is:

Some experts suggest that the exponent should be between 2.3 and 2.7.  The best formula might be:

## Other websites

- U.S. National Center for Health Statistics:
    - BMI Growth Charts for children and young adults
    - BMI calculator for people ages 2–19
    - BMI calculator for people ages 20 and older