According to Centers for Disease Control and Prevention, BMI for children can be calculated by dividing the child's weight by their height squared. When using U.S. systems of measurement (ounces and inches), the result must then be multiplied by 703.
Take the height in inches and square it, that is, multiply it by the same number. Divide the weight in pounds by this result. The final figure must be multiplied by the conversion factor of 703 (This is not necessary for metric standard measures, kilograms and centimeters). It is more difficult to accurately interpret factors of height and weight in children than adults because children are still growing at a generally rapid pace, notes WeightConcern. It is also necessary to take into account age, height and gender. The most convenient tool for this are the several BMI calculators, for children and teenagers, available online as well BMI charts for comparison on sites such as Centers for Disease Control and Prevention. For children, BMI is generally then placed on the BMI percentile chart as the scales for obesity are different in adults and children. However, these measures can only provide a general approximation of BMI. A doctor must be consulted for accurate results.