For years, doctors have used height and weight measurements to assess a child's physical growth in relation to other kids the same age. Now they have another tool: body mass index (BMI).

BMI is a calculation that uses height and weight to estimate how much body fat someone has. Doctors use it to determine how appropriate a child's weight is for a certain height and age.

Calculating BMI

You can use the BMI calculator below to determine your child's BMI, but it's also important to have your doctor perform regular BMI measurements. That way, you'll know the number is accurate and the doctor can discuss the results with you.

Tracking BMI

Starting when your child is 2 years old, the doctor will determine BMI at routine checkups and plot this measurement on a chart against those of other kids the same age.

Because what is normal changes with age, doctors must plot children's BMI measurements on standard growth charts rather than using a universal normal range for BMI as is done with adults. They also use separate charts for boys and girls to account for differences in growth rates and amounts of body fat as the two genders mature.

That information is recorded in your child's medical record, and over several visits the pattern of measurements allows the doctor to track your child's growth.

BMI is particularly helpful for identifying kids and adolescents who are at risk for becoming significantly overweight as they get older. In older kids and teens, there is a strong correlation between BMI and the amount of body fat. So those with high BMI readings — and, probably, high levels of fat — are most likely to have weight problems when they are older.

By identifying these at-risk kids, doctors can monitor their body fat carefully and try to prevent adult obesity through changes in eating and exercise habits.

What the Figures Mean

BMI percentiles show how kids' measurements compare with others the same gender and age. For example, if a child has a BMI in the 60th percentile, 60% of the kids of the same gender and age who were measured had a lower BMI.

BMI is not perfect. For example, it's very common for kids to gain weight quickly — and see the BMI go up — during puberty. Your doctor can help you figure out whether this weight gain is a normal part of development or whether it's something to be concerned about.

Kids can also have a high BMI if they have a large frame or a lot of muscle, not excess fat. And a kid with a small frame may have a normal BMI but too much body fat.

Although BMI is not a direct or perfect measure of body fat, kids at or above the 95th percentile are considered obese, a term doctors use to indicate excess body fat, which increases the risk of weight-related health problems.

Kids who measure at the 85th to 94th percentiles are considered overweight, because of excess body fat or high lean body mass. A child whose BMI is between the 5th percentile to 85th percentile is in the healthy weight range. A child with a BMI below the 5th percentile is considered underweight.

Also, it's important to look at the BMI numbers as a trend instead of focusing on individual numbers. Any one measurement, taken out of context, can give you the wrong impression of your child's growth.

The real value of BMI measurements lies in viewing them as a pattern over time. That allows both doctor and parents to watch a child's growth and determine whether it's normal compared with that of other kids the same age.

While BMI is an important indicator of healthy growth and development, if you think your child may be gaining or losing weight too fast, talk to your doctor.

Reviewed by: Mary L. Gavin, MD
Date reviewed: March 2011