Body Mass Index (BMI) is a value used to determine healthy weight ranges for humans. It was developed by the Belgian polymath Adolphe Quetelet as part of his "social physics" system between 1830 and 1850 (and is therefore also known as the Quetelet Index). It has been used to define the medical standard for the measurement of obesity in several countries since the early 1980s and is the main measure used in the World Health Organization's obesity statistics. It is equal to weight in kilograms divided by the square of height in metres:

BMI = w / h²

(In standard U.S. 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 below 18.5 is underweight and may indicate malnutrition, an eating disorder or other health problem, while a BMI above 25 is overweight and above 30 is considered obese. These range limits apply to adults over 20 years of age. Because BMI does not take into account a person's percentage of body fat, it is possible to have an above-average body weight and BMI, but not be obese. A bodybuilder, for example, may have a BMI greater than 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 1994 U.S. National Health and Dietary Review Survey indicates that 59% of U.S. men and 49% of women have a BMI greater than 25. Extreme obesity - a BMI of 40 or higher - was found in 2% of men and 4% of women. Body mass index calculations are not only for adults, and can be used to identify the increasing number of overweight children. The BMI for children aged 2 to 20 years is calculated as it is for adults, but is interpreted differently. Instead of defining cut-off numbers for being underweight and overweight, as for adults, it is their BMI percentile that is important. For children, a BMI below the 5th percentile is considered underweight and above the 95th percentile is considered overweight. And children with a BMI between the 85th and 95th percentile are considered at risk of becoming overweight.

Recommended BMI thresholds

Underweight - 20 Ideal - 20-24.9 Overweight - 25-29.9 Obese - 30 or more Extremely obese - 40 or more Note: These recommended differences along the linear scale may vary from time to time and from country to country, making comprehensive longitudinal surveys problematic. In 1998, the U.S. NIH brought the U.S. definitions in line with WHO guidelines, lowering the normal/overweight threshold from BMI 27 to BMI 25. This had the effect of redefining approximately 30 million Americans from 'technically healthy' to 'overweight'. The WHO uses the term "pre-obese" where the US uses "overweight". It also recommends lowering the normal/overweight threshold for body types in South East Asia to BMI 23 and expects further studies on different body types to emerge.