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Know Your Child's BMI

Reviewed by Barbara Gordon, RDN, LD
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It can be tricky to tell whether a child or teen is overweight. Since kids are still growing, they need to gain weight. And, it’s normal for them to quickly outgrow their clothes. So, how do you know if your kids’ weights are healthy?

For years, doctors compared height and weight measurements with children of the same age. An alternative measure to these old-fashioned growth charts is body mass index, or BMI. BMI is a number calculated from your child's height and weight, which estimates body fat. Though BMI does not measure body fat directly, it is an indicator of body fatness and disease risk for most kids. Plus, it's an inexpensive and simple method for figuring out whether a child's or teen's weight is healthy.

We can't use an adult BMI chart for kids because the body fat of children changes as they grow. Also, girls and boys differ in body fatness, especially as they mature. So, for kids we need to use sex-specific criteria for evaluating BMI.

BMI-for-age Percentile Charts

BMI-for-age-percentile charts provide measures for boys and girls ages 2 to 20 years:

  • Underweight: BMI is less than the 5th percentile.
  • Healthy weight: BMI is between the 5th and 84th percentiles.
  • Overweight: BMI is between the 85th and 94th percentiles.
  • Obese: BMI is equal to or greater than the 95th percentile.

A few words of caution: Kids who are very athletic and muscular, may have high BMIs. Their body weight may be high because of extra muscle mass. If you are concerned about your child’s BMI, talk with your child’s health care provider. Also, "overweight" and "obese" are medical terms. It is best not to use these terms in front of kids. Instead, talk to your child or teen about being a healthy weight.

Remember the old-fashioned growth chart? Well, the BMI-for-age-percentile charts are in a similar format. It’s a graph with age along the y axis (vertically) and BMIs along the x axis (horizontally). Curves on the graph display the different BMI percentiles.

Two Ways to Calculate Your Child’s BMI

The easiest way to calculate BMI is using an online calculator. The Centers for Disease Control and Prevention provides a free calculator for children and teens. You go online, enter your child’s date of birth, sex, height and weight. Plus, the calculator also asks for the date that you are doing the calculation. That way, if you print out the calculation, you’ll have a record of when you took the measurement. The calculator also explains in easy-to-understand language what the results mean.

You also can calculate BMI the old-fashioned way. And, you don't need to be a math whiz for this option! First, figure out your child’s height and weight. Next, grab a calculator and use one of the following two formulas:

  • Formula 1: BMI = Weight (pounds) / Height (inches) / Height (inches) x 703
  • Formula 2: BMI = Weight (kilograms) / Height (centimeters) / Height (centimeters) x 10,000

It doesn't matter which formula you use, because they both end up with the same BMI. After you do the math, round off the BMI to the nearest whole number. Let's calculate BMI for a boy named Peter. If Peter weighs 129 pounds (59 kilograms) and is 63 inches (160 centimeters) tall, his BMI is 23.

  • Formula 1: BMI = 120 / 63 / 63 x 703 = 23
  • Formula 2: BMI =59 / 160 / 160 x 10,000 = 23

Lastly, pull out your handy BMI-for-age-percentile chart. To download a copy, go to the CDC’s website and select a chart for boys or girls. You also can choose a chart that is in English, Spanish or French and in color or black-and-white. Mark the point on the graph where your child's age and BMI cross. Next, note either the percentile curve that the point lands on or the curves on either side of that point.  Based on the percentile curve, you child’s BMI may be underweight, healthy, overweight or obese.

Tracking BMI Over Time

BMI is the gold standard for screening potential weight problems. However, a single BMI calculation does not tell your child's whole weight story. As kids grow, their body fat changes, which means their BMI will change, too. That is why it's important to track BMI over time.