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Understanding BMI

Body Mass Index Basics


Updated March 15, 2008

In their 2003 policy statement on the Prevention of Pediatric Overweight and Obesity, the American Academy of Pediatrics called on doctors to begin routinely calculating a child's body mass index or BMI to help in the early recognition of childhood obesity.

So now, in addition to plotting your child's height and weight on his growth chart, his BMI should also be charted each year.

What is BMI?

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.

BMI is usually thought of as a tool that is used in treating children who are overweight, however, it can also help determine if children are underweight if they have a low BMI.

Understanding BMI

In adults, once you calculate your BMI, it is rather easy to interpret your results, as a BMI between 25 to 30 is considered overweight and a BMI of 30 and above is considered obese.

Interpreting BMI is a bit more complicated for children though, since you also have to take into account the child's age to figure out the percentile ranking for that BMI from a Girl's BMI growth chart or a Boy's BMI growth chart. This BMI percentile can then help you determine if a child is overweight or at a healthy weight.

Adding to the confusion a little more, instead of classifications for overweight and obese, as we have for adults, the categories for children are that they are either 'at risk for overweight' or 'overweight.'

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

At Risk of Overweight???

The 'At Risk of Overweight' category is especially confusing. Many people interpret that to mean that their child is at a healthy weight and 'might' become overweight later. That category really corresponds to the adult overweight category though, and the child overweight category corresponds to the adult obese category.

To be diplomatic, experts decided to soften the terms in the BMI categories for kids and left out the word obese. To be clear, whether or not they change these classifications, if your child is in the 'At Risk of Overweight' category, you should understand that he is already overweight and you need to make some changes to help him get to a healthier weight.

Why the Controversy?

The big reason for the controversy over BMI categories is that many people do feel the need to be diplomatic and not hurt anyone's feelings when discussing childhood obesity. While it certainly wouldn't be diplomatic to tell a child they are fat, to sidestep the issue and not get families the help they need is also wrong. If the experts don't want to have separate overweight and obese categories, like we have for adults, then a single overweight category for children over the 85th percentile might be a good option.

1Prevention of Pediatric Overweight and Obesity. PEDIATRICS Vol. 112 No. 2 August 2003, pp. 424-430
2CDC. About BMI for Children and Teens.

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