How Can You Calculate Your Child's BMI?

1
Understanding BMI

A doctor measuring a child's weight.
Your pediatrician should measure your child's BMI at each checkup. Photo by Getty Images

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.

Does your pediatrician calculate your child's BMI or body mass index at his yearly well child checks? Do you know how to do it?

BMI

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

In adults, once you calculate your BMI, it is rather easy to interpret your results, as a BMI between 25 to 30 is considered overweight and a BMI of 30 and above is considered obese.

Interpreting BMI is a bit more complicated for children, though, 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.

2
BMI Formula for Kids

A high tech BMI scale.
Instead of going High Tech, most of us calculate BMI the old fashion way. Photo by Getty Images

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.

3
BMI Percentiles for Kids

BMI growth charts can help you figure out your child's BMI percentile.
BMI growth charts can help you figure out your child's BMI percentile. Photo by Vincent Iannelli, MD

In adults, once you calculate your BMI, it is rather easy to interpret your results, as a BMI between 25 to 30 is considered overweight and a BMI of 30 and above is considered obese. Or you are underweight if your BMI is under 18.5.

It's not so simple in kids, though.

In kids, BMI is both age and sex specific.

So in addition to their BMI, you have to know your child's BMI percentile:

  1. Calculate your child's BMI
  2. Find the age and sex appropriate BMI chart for your child
  3. Plot your child's BMI on the BMI chart
  4. Figure out your child's BMI category

You can also expect that your pediatrician will do all of this at your child's next well child checkup.

4
BMI Categories for Kids

BMI percentile categories
Is your child overweight, underweight, or at a healthy weight?. Photo courtesy of the CDC

So 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.

5
Other BMI Tools and Calculators

Is your child overweight?
Is your child overweight?. Photo by Peter Dazeley/Getty Images

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

  • CDC BMI Percentile Calculator - a calculator that provides BMI and the corresponding BMI-for-age percentile on a CDC BMI-for-age growth chart
  • Children's BMI Spreadsheet - an Excel spreadsheet that can be used by schools, child care centers, and other professionals who want to compute BMI on large groups of children

  • Healthy Care for Healthy Kids Obesity Toolkit

  • A digital BMI scale - although these bathroom scales are popular, you still have to input your child's height and they won't calculate a BMI percentile for kids

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