How to Calculate Your Child's BMI

Your child's BMI (body mass index) is an important tool that you can use to determine if your child is overweight, underweight, or at a healthy weight.

Difficulty: Easy

Time Required: It should take less than 10 minutes to calculate your child's BMI.

Here's How:

  1. Weigh Your Child
    Record your child's weight in pounds.
  2. Measure Your Child
    Measure your child's height in inches.
  3. BMI Formula
    Enter your child's weight and height into the BMI formula (use your calculator):
    BMI = [weight / (height x height)] x 703
  1. BMI Percentiles
    Use this calculated BMI and find your child's BMI percentile on a Girl's BMI Chart or Boy's BMI Chart, as appropriate. After finding your child's age on the bottom of the BMI growth chart, follow the line up until it crosses the horizontal line that meets your child's BMI that you just calculated on the side of the growth chart. The curve that intersects these two lines is your child's BMI percentile. For example, on the boy's BMI chart, you can see that a 13-year-old boy with a BMI of 17 is at the 25th percentile for BMI for his age.
  2. BMI Categories
    See what category your child's BMI fits in:
    • 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
  3. Getting Help
    Visit your pediatrician if your child is classified as underweight, overweight, or at risk of being overweight.


    1. Quick and easy ways to get your child's BMI are to use an online BMI calculator, BMI table, a home BMI scale, the Wii Fit game (you still need your child's height though), or visit your pediatrician.

    2. A big criticism of using BMI as a measure for childhood obesity is that it doesn't directly measure body fat. So muscular teens may be incorrectly labeled as having a weight problem. Although most younger children with a high BMI are really overweight and have excess body fat, if you still aren't sure, some other measurements or tests for body fat can be more accurate, including skinfold thickness measurements, waist to hip ratios, and newer scales that actually include body-fat measurements.

    1. See your pediatrician if you are having problems calculating your child's BMI or if your child has a BMI that indicates that he is overweight or underweight.

    2. If you are using the metric system, the formula to calculate BMI is a little different. In this BMI formula, you use kilograms and meters for a child's weight and height and use the formula - BMI = [weight / (height x height)].

    What You Need:

    • your child's height
    • your child's weight
    • a calculator or BMI table
    • a BMI growth chart to look up the percentile ranking for your child's BMI

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