How to Calculate Your Child’s BMI

Your child’s BMI is an important tool that you can use to determine if your child is overweight, underweight, or at a healthy weight.

That’s why the American Academy of Pediatrics has recommended routine screening and tracking of BMI in all children once they are about two years old.


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

Interpreting BMI is a bit more complicated for children than adults since you also have to take into account the child’s age to figure out the percentile ranking for that BMI. This BMI percentile can then help you determine if a child is overweight or at a healthy weight.

To calculate your child’s BMI, you need to:

  1. measure his weight (pounds)
  2. measure his height (inches)
  3. calculate his BMI by dividing your child’s weight by his height squared and multiplying the total by a conversion factor of 703

BMI formula = (weight / (height x height)) x 703

Alternatively, using metric measurements, you would:

  1. measure his weight (kilograms)
  2. measure his height (meters)
  3. calculate his BMI by dividing his weight by his height squared

BMI formula (metric) = weight / (height x height)

You now have to figure out your child’s BMI percentile to see what his BMI actually means:

  • Find the age and sex appropriate BMI chart for your child
  • Plot your child’s BMI on the BMI chart
  • Figure out your child’s BMI category

BMI Categories for Kids

Once you have calculated your child’s BMI and found their percentile, you can figure out if they are:

  • Underweight – BMI less than the 5th percentile
  • Healthy Weight – BMI 5th percentile up to the 85th percentile
  • Overweight – BMI 85th to less than the 95th percentile
  • Obese – BMI greater than or equal to the 95th percentile

Since these categories depend on your child’s age, two kids can have the same BMI and be in different categories.

Other tools to help you figure out if your child is overweight and calculate your child’s BMI and BMI percentiles include: