A body mass index chart can help assess whether or not a child is obese, explains KidsHealth. However, individuals should interpret body mass index calculations with caution, since several factors can lead to inaccurate estimates.
The Centers for Disease Control and Prevention provides a calculator for measuring a child's body mass index, a measurement of body fat percentage based on weight and height, explains the CDC. A parent enters the child's birthdate, date of measurement, gender, height to the nearest 1/8 inch and weight to the nearest 1/4 pound and then clicks Calculate to return the body mass index.
The calculator returns the child's body mass index and his body mass index for age percentile, notes the Centers for Disease Control and Prevention. The percentile shows how the child's weight compares to others of the same height and gender. A child with a percentile of 95 percent or greater is obese.
Body mass index does not perfectly measure body fat. Because muscle adds body weight, muscular children who are neither overweight nor obese may have an artificially high body mass index, according to KidsHealth. Body mass index calculations are particularly prone to error during puberty when children undergo rapid growth.
Physicians may perform childhood body mass index calculations in children ages 2 through 19, writes KidsHealth. For children younger than 2, doctors use weight-for-length charts to assess how a baby's weight compares to its length. Children at the 85th percentile or above are considered overweight.