It is simple using the body mass index (BMI) calculator at kidshealth.org.

Body mass index is a calculation that uses your height and weight to estimate how much body fat you have. Too much body fat is a problem because it can lead to illnesses and other health problems. BMI, although not a perfect method for judging someone’s weight, is often a good way to check on how a kid is growing.

www.kidshealth.org has one of the best tools for calculating BMI specifically for young folks.