Body Mass Index (BMI) is calculated by taking into account a child's height and weight. BMI is used to determine approximate body fat; it varies by age and sex. Often, BMI is plotted on a BMI-for-age chart that provides percentile rankings comparing the child to others of his same sex and age. The chart will also indicate whether or not's the child's weight is considered healthy. The American Academy of Pediatrics recommends that doctors calculate a child's BMI beginning at age 2. Healthy weight ranges change with each month of a child's life.
The formula for calculating BMI is weight (lb)/height (in) squared x 703. An online calculator is available from the Centers for Disease Control (see Resources).
BMI growth charts account for a variety of factors, including weight, length, stature and head circumference. They can be also be obtained from the CDC (see Resources).
BMI in children generally ranges from 12 to 34. Healthy BMIs are dependent on the child's exact age and height at any given moment. Thus, it is impossible to say exactly what a healthy BMI is.
What BMI Determines
BMI is an indicator of how much fat is in the body, and can be used to determine if a child is underweight, overweight or obese.
Percentile rankings are used to determine whether or not the child is at a healthy BMI and weight. If the BMI percentile is under 5 per cent, the child is underweight. If it is anywhere from 5 to 85 per cent, the child is considered to be at a healthy weight. If the BMI percentile is 85 to 95 per cent, the child is considered overweight, and anything above the 95 percentile is considered obese.
Other Tests to Determine Body Fat
While BMI calculations are used as a screening tool, other tests are often needed to determine fat content in the body. Taking a skin-fold measurement is one way in which a doctor might assess body fat. Also, dual energy x-ray absorptiometry and underwater weighing may be used to measure body fat.