BMI Norms for Children

Body Mass Index (BMI) is a measurement used to assess if a person is obese, overweight, average size or underweight. BMI is a number that is calculated using the height and weight of an individual. In children, BMI norms are based on the size and growth rates of the average child and are often referred to as BMI-for-age, according to the Centers for Disease Control and Prevention (CDC).

Males 2 to 20 Years

According to the CDC, a 2-year-old male is in the 50th percentile with a BMI of approximately 16.6. Between the ages of 2 and 6, the BMI is expected to lower slightly each month with a low point around 15.4. Around age 6, the BMI is expected to begin increasing gradually until age 20 when a young man's BMI of 23 is in the 50th percentile.

Females 2 to 20 Years

A 2-year-old female's BMI is in the 50th percentile with a BMI of 16.4. This number, as in males, is expected to decrease until about age 6. Once the young girl reaches this age, the 50th percentile BMI is approximately 15.1 and begins to increase. Between the ages of 8 and 12, there is an increase of nearly two points leading to the point where the BMI of males and females has the greatest difference. By age 20, a female's BMI is in the 50th percentile at 21.7.

Concerns for All Children

For children, BMI is used as a tool to screen for obesity and other weight abnormalities. It must be used in conjunction with a physician assessment as it is not a sole indicator of weight problems in children because of the variations of growth in children. The CDC and the American Academy of Pediatrics (AAP) both recommend the use of BMI to screen for overweight and obesity in children beginning at 2 years old. Other assessments used are skinfold thickness measurements and evaluations of physical activity, diet and family history of obesity.