Body mass index (BMI), as defined by the Centers for Disease Control and Prevention (CDC), is "a number calculated from a person's weight and height. BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems." The formula for calculating BMI is the same for adults and children, but the method of interpreting the results differs.
Calculating BMI
BMI is calculated by dividing weight in pounds by height in inches squared and then multiplying by a conversion factor of 703. If using metric measurements, BMI is calculated by dividing weight in kilograms by height in meters squared. The CDC provides online BMI calculators for both adults and children.
Interpreting BMI in Adults
When used to assess weight status in adults 20 years old or older, BMI is interpreted with standard weight status categories that are the same for all age groups and for both men and women. Adults with a BMI below 18.5 are considered underweight, those with BMIs 18.5 to 24.9 are considered normal weight, those with a BMI between 25.0 and 29.9 are considered overweight, and those with BMIs of 30.0 and above are considered obese.
Interpreting BMI in Children and Teens
When interpreting BMI numbers for children and teens, adjustments are made using BMI age- and sex-specific percentiles, because the amount of body fat changes with age and because the amount of body fat differs between boys and girls. The CDC provides BMI-for-age growth charts to use in translating BMI numbers into percentiles for a child's sex and age. Children in the less than the 5th percentile are considered underweight, those in the 5th percentile to less than the 85th percentile are considered healthy weight, those in the 85th to less than the 95th percentile are considered overweight, and those equal to or greater than the 95th percentile are considered obese.
BMI as an Indicator of Body Fatness
Although the BMI number is strongly correlated with body fatness, there is a good deal of variation depending on sex, age and race. For instance, at the same BMI, women and older people tend to have more body fat. In addition to BMI, the National Heart, Lung, and Blood Institute recommends also looking at waist circumference and other risk factors when assessing the likelihood of an individual developing overweight- or obesity-related diseases.
Muscularity and BMI
Individuals with a high level of muscularity but not a high level of body fat, such as athletes, may have a high BMI. The CDC notes that "although some people with a BMI in the overweight range (from 25.0 to 29.9) may not have excess body fatness, most people with a BMI in the obese range (equal to or greater than 30) will have increased levels of body fatness."
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