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.


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.

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.

This is the formula:


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

A man whose weight is 172 lb and height is 72 in 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.