The standard body mass index may not be as accurate for children as it is for adults, explains WebMD. Because boys and girls grow and develop at different rates that affect body fat, the figure is not always accurate, state the Centers for Disease Control and Prevention. While BMI is calculated solely on height and weight in adults - a person's weight in kilograms divided by the square of height in meters - kids' BMI is age and sex-specific.
As a result, when health care professionals discuss the BMI of children, they often refer to percentiles, which indicate how the BMI compares to that of other children of the same age and gender, states WebMD. For example, a 7-year-old boy with a BMI in the 65th percentile has a higher BMI than 65 out of 100 boys of his age, explains WebMD.
Standard children's percentiles indicate those below the fifth percentile to be underweight, while those from the fifth through the 85th percentile are generally considered to be of healthy weight. Boys and girls falling between the 85th and 95th percentile are considered overweight and those above the 95th percentile are categorized as 'obese,' according to information provided by the Centers for Disease Control.