Body mass index

A graph of body mass index is shown above. By lining up weight and height you can find out in which range an adult person falls. Left of normal range means too thin, to the right of normal means too fat. Dashed lines give a rough idea of how much too thin or how much too fat a person is.
Based on World Health Organization data here.

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 called the Quetelet Index.

History

The BMI was invented by Adolphe Quetelet between 1830 and 1850. BMI is still sometimes called the Quetelet Index.

A scientist called Ancel Keys first used the name body mass index in 1972. He wrote that governments should measure people's BMI to find out whether their people are too fat or too thin.

Calculation

To find somebody's BMI:

• measure their weight (body mass) in kilograms (if in imperial pounds multiple by 0.45359237 to find kilograms)
• measure their height in meters (if in inches multiple by 0.0254 to find meters)
• divide their weight by the square of their height

This is the formula:

$\mbox{BMI} = \frac{weight}{height ^2}$

For example, a man whose weight is 78 kg and height is 1.83 m has a BMI of:

$\mbox{BMI} = \frac{78}{1.83 ^ 2} = 23.3$

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.

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. Another way to measure body weight and size is the Body Volume Index.

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

$\mbox{BMI} = \frac{weight}{height ^3}$

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

$\mbox{BMI} = \frac{weight}{height ^{2.4}}$

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