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About BMI for Children and Teens

By Stefanie Hamilton

Most health care professionals use body mass index (BMI) to screen for overweight and obesity in children and teenagers. Though BMI is not a diagnostic tool and doesn't measure actual body fat, the index has been determined to have a strong correlation to overweight and obesity.

How is BMI Calculated?

Body mass index is calculated with a formula that uses height and weight. Using pounds for weight and inches for height, the formula is weight divided by height squared, multiplied by 703. This number is then plotted on a chart and assigned a percentage.

·        A BMI that falls under the 5^th percentile is considered underweight.

·        BMIs within the 5^th to 85^th percentile are considered healthy weight.

·        A BMI between the 85^th and 95^th percentile is considered overweight.

·        BMIs over the 95^th percentile are considered obese.

For young people, an age-and-gender-adjusted BMI chart is used. Age and sex are taken into consideration when determining BMI for young people for two main reasons:

1.      Body fat percentages change with age.

2.      Girls typically have more body fat than boys do.

 This means that two children (of different ages or genders) can have the same BMI value, yet one can be considered overweight or obese while the other is not.

Does a High BMI Score Mean My Child is Obese?

Not necessarily. Though BMI is a great screening tool, it's not a diagnostic. This means that the score can indicate overweight or obesity, but should never be used to make a formal diagnosis.

For example, some children are more muscular than others, and since muscle weighs more than fat, a child with more muscle mass with have a higher BMI, but isn't necessarily overweight or obese. In addition, most kids gain a significant amount of weight just before a growth spurt. If BMI is calculated during this time, it won't be an accurate representation.

If your child has a high body mass index, he should be examined by a pediatrician who can conduct additional tests. One of the more popular, and accurate, body fat assessments is done using skinfold measurements. With this technique, skin is “pinched” and the thickness of the skin is measured. Measurements are taken at various points on the body and used to calculate percentage of body fat.

Using BMI to Improve Your Child's Health

If it is determined that your child is overweight or obese, a pediatrician can help develop a weight loss strategy that includes exercise and changes in diet.

Remember, though, that a child's diet and exercise regimen should never be changed without consulting with your pediatrician first. Children's bodies are still developing, and as such need certain nutrients and vitamins (including fat) in order to develop well.

Your child's doctor can help put together a program that will be easy to follow, will promote continued proper development, and will help your child achieve and maintain a healthy weight.