Updated June 10, 2014.

Although it is quite easy to calculate a child's body mass index or BMI using an online BMI calculator or look it up on a BMI chart, you could also calculate it yourself with the BMI formula.

This is not always practical though, especially you have to calculate the BMI of a lot of children, like in a nurse's office at school or for a busy Pediatrician.

BMI Formula

If you are using pounds and inches for a child's weight and height, the formula to calculate BMI is:

BMI = [weight / (height x height)] x 703

Again, in this BMI formula, the child's weight is in pounds and his height is in inches and you basically 'calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703.'

BMI Formula (Metric Units)

If you are using the metric system, the formula to calculate BMI is a little different. In this BMI formula, you use kilograms and meters for a child's weight and height:

BMI = [weight / (height x height)]

So using the metric system, 'the formula for BMI is weight in kilograms divided by height in meters squared.'

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 from a Girl's BMI Chart or a Boy's BMI Chart. This BMI percentile can then help you determine if a child is overweight or at a healthy weight.

See our article on Understanding BMI for more information.

References:
¹CDC. About BMI for Children and Teens.