Body mass index (BMI), expressed as weight/height2 (kg/m2), is most often used to define overweight and obese conditions. Using a growth chart, your child’s pediatrician will calculate your child's percentile, and how he/she compares with other children of the same sex and age. For example, if you are told your child’s BMI is in the 80th percentile, that means that, compared with other children of the same sex and age, 80 percent have a lower BMI.

• Between 85th and 94th percentiles — overweight
• 95th percentile or above — obese
• More than 1.2 times the 95th percentile — extremely obese

The BMI calculation does not take into account factors like body frame or muscularity, which can mean a tall, big-boned teenager who plays football and works out regularly may have a high BMI, but not a lot of excess body fat. So your child’s pediatrician will also consider your child's growth and development, your family's history of obesity and weight-related health problems -- such as diabetes -- as well as your child's eating habits, calorie intake, activity level, and overall health when diagnosing childhood obesity.