How to Calculate Your Child's BMI

Use this important, quick screening index to identify your kid's BMI to determine if your child is underweight, overweight, or obese.

The numbers of overweight kids and childhood obesity rates are not going down.
Photo: kwanchai.c/Shutterstock

A number of methods can be used to determine if someone is overweight or obese, but a fast way to check if you or your child is on target is to figure out body mass index (BMI).

BMI for children (also referred to as BMI-for-age) is gender and age-specific from two to 20 because girls and boys differ in body fat content as they mature.

To calculate your child's BMI:

1. Determine your child's weight in kilograms. If you only know it in pounds, you can use the easy calculations below.

  • Take your child's weight in pounds. (For example, 37 pounds, 4 ounces becomes 37.25.)
  • Divide that number by 2.2. (So, 37.25 divided by 2.2 equals 16.9 kilograms.).
  • The resulting number is your child's weight in kilograms.

2. Determine your child's height in meters. If you only know it in inches, you can use the easy calculations below.

  • Take your child's height in inches. (For example, if your child is 41 1/2 inches tall, it turns into 41.5.)
  • Multiply that number by .0254. (So, 41.5 multiplied by .0254 equals 1.05 meters.)
  • The resulting number is your child's height in meters.

3. With your child's weight in kilograms and height in meters, you can plug those numbers into the BMI formula to get your child's body mass index.

  • First, multiply the height number by itself. Using our example, this would be 1.05m x 1.05m = 1.10m.
  • Then, take the weight in kilograms and divide it by the height squared (the number you got in part 3a.). Again using our example, it would be 16.9kg divided by 1.10m, yielding a BMI of 15.3.

Once you have figured out your child's index, you can refer to the BMI printable chart to find out what percentile your child is in:

Healthcare professionals use the following established percentile cutoff points to identify underweight and overweight ranges in children:

  • Underweight: BMI-for-age < 5th percentile
  • At risk of overweight: BMI-for-age 85th percentile to < 95th percentile
  • Overweight: BMI-for-age > 95th percentile

BMI decreases during the preschool years, then increases into adulthood. The percentile curves show this pattern of growth. If your child has a body-mass index (BMI) that places him above the 95th percentile based on his age, sex, weight, and height, he's at high risk for type 2 diabetes; if it is 85 percent or higher, ask your pediatrician whether your child should be tested for diabetes.

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