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What is BMI?

Overweight and obesity generally are gauged through a formula called the Body Mass Index (BMI). BMI is calculated by dividing a person’s weight in kilograms by their height in meters squared. People can be categorized as being underweight, at a normal weight, overweight or obese depending on their BMI (Exhibit 2).

BMI can be inaccurate because it is often calculated using self-reported figures for height and weight; participants tend to underestimate their weight and overestimate their height. BMI calculations also have certain limitations. They do not take into account muscle mass, meaning that some people, particularly athletes, may be classified as overweight despite lacking significant amounts of body fat. Despite these drawbacks, however, BMI is generally considered to be the best tool available for weight categorization, and is used by the U.S. Centers for Disease Control and Prevention (CDC).

BMI can be inaccurate because it is often calculated using self-reported figures for height and weight

In children, the relationship between BMI and obesity is somewhat more complex. Like adult BMI, child BMI depends on weight and height. But BMI in children is also defined by age and gender, since a child’s amount of body fat varies with these two factors. To account for differences in age and gender, the CDC has developed BMI-for-age growth charts that classify children based on how they compare with other children of their age and gender (Exhibit 3).30

Exhibit 2

BMI Categories for Adults

Weight Category BMI
Underweight Less than 18.5
Normal Weight 18.5 to 24.9
Overweight 25.0 to 29.9
Obese 30.0 or more

Exhibit 3

BMI Categories for Children

Weight Category BMI
Underweight Less than the 5th percentile
Healthy Weight 5th percentile to less than the 85th percentile
Overweight 85th to less than the 95th percentile
Obese Equal to or greater than the 95th percentile

Source: U.S. Centers for Disease Control and Prevention.

End Notes

  • 30 U.S. Centers for Disease Control and Prevention, “About BMI for Adults,” Arno J. Krul, Hein A.M. Daanen and Hyegjoo Choi, “Self-Reported and Measured Weight, Height and Body Mass Index (BMI) in Italy, the Netherlands and North America,” European Journal of Public Health (January 20, 2010); and U.S. Centers for Disease Control and Prevention, “About BMI for Children and Teens,” (Last visited January 5, 2011.)
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