The body mass index (BMI) is a calculated number used to determine healthy weight ranges for humans. It was developed by the Belgian polymath Adolphe Quetelet in the course of working out his system of ‘social physics’, between 1830 and 1850 (and is therefore also known as the Quetelet Index). It has been used to define the medical standard for obesity measurement in several countries since the early 1980s, and is the main measure employed in World Health Organization obesity statistics. It is equal to the weight in kilograms, divided by the square of the height in metres:
BMI = w/h²
(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.
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 distinctions along the linear scale may vary from time to time and country to country, making global, longitudinal surveys problematic. In 1998, the US NIH brought US definitions into line with WHO guidelines, lowering the normal/overweight cut-off from BMI 27 to BMI 25. This had the effect of redefining approximately 30 million Americans, previously ‘technically healthy’ to ‘technically overweight’. The WHO uses the term ‘pre-obese’ where the USA uses ‘overweight’. It also recommends lowering the normal/overweight threshold for South East Asian body types to around BMI 23, and expects further revisions to emerge from clinical studies of different body types.