How to Calculate Body Mass Index for Children

by Contributing Writer

Overview

For more than a century, the Body Mass Index (BMI) has been a useful tool for detecting potential weight problems and the associated health risks. The BMI is simply a ratio of height to weight, and is easily calculated. What is important, however, is to understand what is considered a healthy range and what is considered unhealthy. The BMI for children is calculated the same way as that for adults, but the numbers are compared to norms for children of the same age and gender.

Step 1

Weigh the child and multiply this number by 703.
Step 2

Measure the child's height in inches, and square this number (multiply it by itself).
Step 3

Divide the first number by the second number. This will produce the child's BMI.
Step 4

Compare this figure to a chart of BMI figures for children to see what percentile your child fits in. (See link below to charts maintained by the Centers for Disease Control and Prevention.) A child with a percentile higher than about 85 is considered overweight, while a child with a percentile below 15 is at risk of being underweight.

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Note that BMI charts often list metric units for height and weight. Do not let this confuse you, as the percentiles and BMI numbers remain the same. The reason you multiply the weight by 703 is to adjust for this difference. The BMI is a good general indicator, but it does not accurately measure body fat. Furthermore, "overweight" should not necessarily be construed to mean obese. Note that the same percentile will have different implications for boys than for girls.
Note that BMI charts often list metric units for height and weight. Do not let this confuse you, as the percentiles and BMI numbers remain the same. The reason you multiply the weight by 703 is to adjust for this difference.
The BMI is a good general indicator, but it does not accurately measure body fat. Furthermore, "overweight" should not necessarily be construed to mean obese.
Note that the same percentile will have different implications for boys than for girls.

Overview