"At risk of overweight" and "overweight" are the terms preferred to refer to children and adolescents whose excess body weight could pose medical risks. Due to potential negative connotations associated with the term "obesity," "overweight" is preferred. Using the 2000 CDC growth charts, at risk of overweight for ages 2 to 20 years is defined as a Body Mass Index (BMI)-for-age between the 85th and the 95th percentiles. Overweight in children is defined as a BMI-for-age at or above the 95th percentile on the charts. BMI is weight in kilograms divided by height in meters squared (kg/m²).

While BMI is commonly used to evaluate overweight and obesity in adults, only recently has it been recommended to screen children and adolescents. An advantage of using BMI-for-age is that it can be used continuously from age 2 years through adulthood. BMI is used differently to define overweight in children and adolescents than it is in adults.