Body Mass Index in Children
Checking On Your Child's BMI Score in Comparison to Other Children
Mar 26, 2008
Once your son reaches his second birthday, your pediatrician will begin to evaluate his body mass index. This is a calculation comparing you son’s height and weight to estimate the percentage of body fat.
Body Mass Index Charts
Height and weight are indexed using standard charts based on age and gender. Charts were developed by the National Center for Health Statistics (NCHS) and adopted in 2000 and replaced the 1977 weight-for-stature charts. BMI is calculated for both males and females but on separate charts, since boys have less natural fat than girls.
Calculations - Understanding the Forumulas
BMI for children is calculated using the same basic formula used for adults, but children are compared to others of the same age and height with weight used to calculate body fat as compared to other children in the same age group. In other words, you won’t see a chart that says that your son should weigh a certain number of pounds when he measures a certain number of inches. Instead, you will see a graph and can see where your son ranks in terms of other children.
BMI is calculated by taking your child's weight in pounds and dividing by height in inches squared and then multiplying by a conversion factor of 703. In equation form, that's 703*(weight /(height*height)). If your son weighs 35 pounds and stands 36 inches tall, that would be 703*(35 / (36*36)), for a BMI of 18.9.
Your son’s BMI is then plotted on a chart based on his gender and age. Body composition varies during childhood, so a comparative chart provides a baseline without the need to create different charts for every age group throughout childhood. A BMI of 18.9, for example, might be low or average depending on your son’s age. It depends on how he compares to other boys of the same age.
Plotting Your Child's Weight
In looking at a BMI chart, you will see curves plotting where same age children rank from lowest to highest. There will be a 50th percentile mark indicating the average and then percentage values lower and higher. Most boys will fall somewhere in the middle of the curve as compared to other boys, but some will be at the low and high end. This is where doctors will have some concerns.
BMI Categories for Children
Underweight – BMI below the 5th percentile
Healthy Weight – 5th to 85th percentile
At Risk for Overweight – 85th to 95th percentile
Overweight – 95th percentile and up
Follow Up on Results
If your son is under or overweight as compared to peers, your pediatrician will evaluate him to determine if there are problems. In some cases, BMI does not accurately reflect general health. For instance, your son may be very muscular and have a high BMI score. Muscle is denser than fat, so your son’s weight numbers could skew the results if he’s more muscular than other little boys.
On the other hand, obesity has become a nationwide problem. Identifying overweight children and those at risk allows doctors to help coordinate plans to address concerns early on. You can, along with your doctor, work on diet and exercise changes that can head off problems starting early on.
