Body mass index (BMI)

What is the formula for calculating body mass index (BMI)?

BMI is a calculation of a person’s weight in relation to his height.

To calculate your child’s BMI, you will first need to know his current weight. Since his height must also be taken into consideration, you will need to know his height in inches. Although you can do this at home, your child’s pediatrician can obtain the most accurate measurements during a routine doctor’s visit.

Once you have the number for both height in inches and weight, follow these steps:

• Multiply his height in inches by itself (or height squared). So, let’s say your child is 48 inches tall (4 feet). You would multiply 48 x 48 to equal 2304. Now you have your child’s height number (2304).

• The second number you need is your child’s weight. For example, let’s say your child weighs 75 pounds.

• The third number you need is 703. Because BMI calculations are based on the metric system, the number 703 is used to standardize BMI measurements.

Now you plug in these numbers to the BMI formula:  BMI = (weight) divided by (height squared) x 703. 

Here’s how it would look using the numbers from the example above: BMI = (75) divided by (2304) X 703. Rounded up, the answer would be 22.9.

Your child’s pediatrician would plot the number 22.9 on a graph to figure out what BMI percentile he falls into.

Learn about how BMI percentiles are interpreted in the Tests section.