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 20 is underweight and a BMI less than 18 indicates malnutrition or other health problem, while a BMI greater than 26 is overweight and above 30 is considered obese.
These range boundaries apply to 25- to 34-year-olds and increase one point per decade of age, so BMIs of 23 to 28 are normal for persons 55 to 65 years of age.
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.