The Body mass index reference article from the English Wikipedia on 24-Apr-2004
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Body mass index

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The body mass index (BMI) is a calculated number used to determine healthy weight ranges for humans and has been used to define the medical standard for obesity measurement in several countries since the early 1980s. It is equal to the weight in kilograms, divided by the square of the height in metres. (In U.S. customary units, it is 703.07 times the weight in pounds, divided by the square of the height in inches.)

The exact index values used to determine weight categories vary from authority to authority, but in general a BMI less than 18.5 is underweight and may indicate malnutrition, an eating disorder, or other health problem, while a BMI greater than 25 is overweight and above 30 is considered obese. These range boundaries apply to adults over 20 years of age.

Since BMI does not take into account a person's body fat percentage, it is possible to have an above average body weight and BMI, but not be obese. A bodybuilder, for example, can have a BMI above 30 because of a high percentage of muscle mass. If they also had a low percentage of body fat, they would not need to lose weight to be healthy.

The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of American men and 49% of women have BMIs over 25. Extreme obesity -- a BMI of 40 or more -- was found in 2% of the men and 4% of the women.

Body mass index calculations are not just for adults though, and they can be used to identify the growing number of overweight children. BMI for kids aged 2 to 20 years is calculated just like it is for adults, but it is interpreted differently. Instead of set cutoff numbers for being underweight and overweight, like for adults, it is their BMI percentile that is important.

For children, a BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is overweight. And children with a BMI between the 85th and 95th percentile are considered to be at risk of becoming overweight. You can easily determine what a child's BMI means by plotting it on a growth chart or using an online BMI calculator.