Body mass index
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Body mass index (or BMI) is a measure which shows whether a person's weight is appropriate for their height. It is also called the Quetelet Index, after its creator Adolphe Quetelet. It was invented between 1830 and 1850. BMI is used by the World Health Organisation and by doctors and other health workers today even though it is more than 150 years old.
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[change] Calculation
BMI is worked out by dividing a person's body weight by the square of their height, and is almost always expressed in the unit kg / m2, which is therefore often left out. This is the formula.
[change] Overweight or not?
The BMI can be used as a guide to whether someone is overweight. These figures are not to be used for small children who are still growing. Different people use different boundaries and they are only ever a guide, but these are often used:
[change] Men
- Underweight: less than 20 (<20)
- Ideal: between 20 and 25 (>=20 but <25)
- Overweight: between 25 and 30 (>=25 but <30)
- Obese: 30 or more (>=30)
[change] Women
- Underweight: less than 18 (<18)
- Ideal: between 18 and 25 (>=18 but <25)
- Overweight: between 25 and 30 (>=25 but <30)
- Obese: 30 or more (>=30)
[change] Variations
The numbers should only be taken as a guide. Different places may apply different numbers.
In 1998, the US National Institutes of Health brought US definitions into line with WHO guidelines. They reduced the normal/overweight cut-off from BMI 27.8 to BMI 25. About 30 million Americans who were 'normal' before, were suddenly 'overweight'. The World Health Organization uses the term "pre-obese" where the USA uses "overweight".
For Asian body types the WHO says to use BMI 23, instead of 25. More studies need to be done, to set more correct numbers.
For Asians, the new cut-off BMI index for obesity is 27.5 compared with the normal WHO figure of 30. An Asian adult with a BMI of 23 or greater is now considered overweight and the normal range is 18.5-22.9.
