Reinventing Health

Body Mass Index (BMI) for Age Initiative

What is BMI?

In their 2003 policy statement on the Prevention of Pediatric Overweight and Obesity the American Academy of Pediatrics called on physicians to begin routinely calculating a child's body mass index or BMI to help in the early recognition of childhood obesity. Overweight is an increasingly prevalent pediatric health problem but is underdiagnosed.

BMI is an alternative for direct measures of body fat. BMI is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems. It is a useful indicator and should also be charted each year. Despite recommendations endorsing the use of body mass index (BMI) to identify overweight children, clinicians seldom use it.

BMI is calculated with a child's height and weight using a simple formula, a BMI calculator, or by looking it up on a BMI wheel or BMI tables. Although it doesn't measure body fat, BMI can be used to determine if a child is overweight or underweight if they have a low BMI.

Understanding use of BMI in children

In adults, once BMI is calculated, it is rather easy to interpret the results as a BMI between 25 to 30 is considered overweight and a BMI of 30 and above is considered obese. The adult calculator provides only the BMI number and not the BMI age and gender specific percentile that is used to interpret BMI and determine the weight category for children and teens. It is not appropriate to use the same categories for adults to interpret BMI numbers for children and teens.

Interpreting BMI is a bit more complicated for children than for adults, requiring adjustment for the child's age to determine the percentile ranking from a Girl's BMI growth chart or a Boy's BMI growth chart. This BMI percentile can then be used to determine if a child is overweight or at a healthy weight. Adding to the specificity the categories for children are that they are either 'at risk for overweight' or 'overweight.'

Age and gender are considered for children and teens for two reasons:

The amount of body fat changes with age. (BMI for children and teens is often referred to as BMI-for-age.)

The amount of body fat differs between girls and boys.

BMI Categories for Kids

Underweight - BMI less than the 5th percentile

Healthy Weight - BMI 5th percentile up to the 85th percentile

At Risk of Overweight - BMI 85th to less than the 95th percentile

Overweight - BMI greater than or equal to the 95th percentile

What is a BMI percentile?

After BMI is calculated for children and teens, the BMI number is plotted on the CDC BMI-for-age growth charts (for either girls or boys) to obtain a percentile ranking. Percentiles are the most commonly used indicator to assess the size and growth patterns of individual children in the United States. The percentile indicates the relative position of the child's BMI number among children of the same sex and age. The growth charts show the weight status categories used with children and teens (underweight, healthy weight, at risk of overweight, and overweight).

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