Body mass index, or BMI, is calculated using a person's height and weight. Unlike the adult BMI, the calculation for children and teens includes sex and age. The Centers for Disease Control and Prevention recommends using the BMI to screen children 2 years of age and older for excess weight and obesity. Although BMI is not an accurate measure of body fat, the calculation is an alternative and approximate measure of body fat for evaluation of potential health problems.
BMI-for-Age
The amount of a child's body fat changes with age and the amount of body fat children carry differs for boys and girls. According to the CDC, it is not possible to list normal or average weights for children and teens since the ranges change, as often as monthly, with increases in height. A child's BMI is evaluated based on the boys' or girls' BMI-for-age growth charts to determine where that child's weight places him in relation to other children in the United States.
Percentile
The BMI-for-age percentile ranks your child's weight based on the CDC's growth charts and matches your child's BMI-for-age with children of the same age and sex. Percentiles are used in the US to chart size and growth patterns of children and teens. The CDC growth charts display weight status categories. For instance, the healthy weight category is in the 5th percentile to less than the 85th percentile, while the overweight category describes the 85th to less than the 95th percentile.
Height and Weight
Measure your child's height to the 1/8th inch or 0.1 cm by first removing your child's shoes and having him stand on uncarpeted flooring, against a wall with his shoulders level. Weigh your child after he has removed his shoes and clothing. Use a digital scale placed on uncarpeted floor and record weight to the nearest decimal fraction, such as 55.5 lb.
BMI Calculation
The CDC provides a BMI calculator on its website. After entering your child's date of birth, date of measurements, height, weight and sex, the calculator gives a BMI-for-age percentile based on the information. For instance, a 12-year-old child who is 4 feet 3 inches tall and 95 lb. has a BMI of 25.7, which ranks her in the 95th percentile for girls of her age.
Meaning and Use
Health care professionals use the BMI-for-age percentile to interpret a child's BMI; however, the calculation is just one tool and additional testing and evaluation are necessary. For instance, calculation that ranks the 12-year-old girl in the 95th percentile includes notes that state the girl might be obese and should see a doctor to assess potential health problems related to her weight. The BMI calculator provides additional information and recommendations based on the child's BMI-for-age percentile.
Tip
Height and weight measurements taken recently by a health care professional are best for use in your child's BMI calculation according to the CDC.
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