How to Calculate BMI in Children

How to calculate BMI in children. Tetra Images - Mike Kemp/Brand X Pictures/Getty Images

You probably know that body mass index (BMI) is considered a more reliable indicator of whether your weight is in a healthy range than the number on the scale suggests. But you may have wondered whether this applies to your children as well. In other words, does BMI accurately reflect a child’s weight status? The answer is: Yes—but there are caveats. 

Firstly, the BMI isn't a great indicator of health for very athletic (ie. muscular) children, because muscle is heavier than fat.

Secondly, it’s important to understand that BMI does not measure body fat directly. It reflects a child’s weight relative to his height, just as it does for adults. In the case of kids, however, there’s a complicating factor: Rather than being based purely on the weight-to-height ratio, BMI in children is age- and gender-specific so it is often referred to as BMI-for-age.

The BMI charts that are used for kids show how your child is growing compared to other boys or girls of the same age. After BMI is calculated (to figure out your child’s BMI, click here), the BMI number is plotted on a BMI-for-age growth chart (specifically for girls or boys), designed by the Centers for Disease Control and Prevention. These growth charts depict percentiles that reflect the average size and growth patterns of children in the U.S., year by year, according to gender. So the plot point where your child’s BMI falls indicates where your child ranks among his or her peers of the same age and same sex.


These growth charts also show where their weight status falls in terms of the following categories: underweight, healthy weight, overweight, and obese. The underweight category means a child is under the 5th percentile for weight. The healthy weight bracket encompasses the 5th percentile to just under the 85th percentile.

The overweight category means a child is somewhere between the 85th and the 95th percentile, and the obese category means a child is at the 95th percentile or higher. Again, the percentiles and the corresponding categories vary, depending on whether your child is a girl or a boy.

There are several reasons why “overweight” is defined differently for kids than it is for adults. For one thing, children are still growing and boys and girls typically grow at different rates. Plus, the amount of body fat a child has naturally changes with age—and it changes at different rates for boys and girls. What’s more, healthy weight ranges change on a monthly basis for kids of all ages and both genders, and they change as height increases, too. So while a child’s weight may put him in the overweight category one year, if he has a growth spurt the following year and gains just a few pounds, his BMI could land him in the healthy weight category instead. By contrast, adults don’t continue to grow in height so this approach just doesn’t make sense for us.


The CDC and the American Academy of Pediatrics (AAP) recommend using BMI to screen for overweight and obesity in children and teens between the ages of 2 and 20. The truth is: Some parents may not recognize or be willing to admit if their child is in danger of becoming obese. By measuring a child’s BMI every year, pediatricians and family doctors can track a child’s weight changes, give dietary and exercise advice as needed, and discuss with parents the health risks that can occur if a child becomes severely overweight. These are just a few reasons why the BMI calculator and BMI-for-age charts really are valuable tools.


Centers for Disease Control and Prevention. About BMI for Children and TeensAccessed online August 12th, 2014.
Centers for Disease Control and Prevention. Using the BMI-for-Age Growth Charts. Accessed online August 12th, 2014.
National Heart, Blood, and Lung Institute. Calculate Body Mass Index. Accessed online August 12th, 2014.
American Academy for Pediatrics. Obesity and BMI. Accessed online August 12th, 2014.
Body mass index and percentage of body fat as indicators for obesity in an adolescent athletic population. Etchison WC, Bloodgood EA, et al. Sports Health. 2011 May;3(3):249-52.

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