What is BMI?

Body Mass Index (BMI) is a number calculated from a child’s weight and height. BMI is a reliable indicator of body fatness for most children and teens. BMI does not measure body fat directly, but research has shown that BMI correlates to direct measures of body fat, such as underwater weighing and dual energy x-ray absorptiometry (DXA).¹ BMI can be considered an alternative for direct measures of body fat. Additionally, BMI is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems.

For children and teens, BMI is age- and sex-specific and is often referred to as BMI-for-age.

What is a BMI percentile?

After BMI is calculated for children and teens, the BMI number is plotted on the CDC BMI-for-age growth charts (for either girls or boys) to obtain a percentile ranking. Percentiles are the most commonly used indicator to assess the size and growth patterns of individual children in the United States. The percentile indicates the relative position of the child’s BMI number among children of the same sex and age. The growth charts show the weight status categories used with children and teens (underweight, healthy weight, at risk of overweight, and overweight).

BMI-for-age weight status categories and the corresponding percentiles are shown in the following table.

How is BMI used with children and teens?

BMI is used as a screening tool to identify possible weight problems for children. CDC and the American Academy of Pediatrics (AAP) recommend the use of BMI to screen for overweight in children beginning at 2 years old.

For children, BMI is used to screen for overweight, at risk of overweight, or underweight. However, BMI is not a diagnostic tool. For example, a child may have a high BMI for age and sex, but to determine if excess fat is a problem, a health care provider would need to perform further assessments. These assessments might include skinfold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.

How is BMI calculated and interpreted for children and teens?

Calculating and interpreting BMI using the BMI Percentile Calculator involves the following steps:

1. Before calculating BMI, obtain accurate height and weight measurements.

2. Calculate the BMI and percentile using the Child and Teen BMI Calculator. The BMI number is calculated using standard formulas.

3. Review the calculated BMI-for-age percentile and results. The BMI-for-age percentile is used to interpret the BMI number because BMI is both age-and sex-specific for children and teens. These criteria are different from those used to interpret BMI for adults — which do not take into account age or sex. Age and sex are considered for children and teens for two reasons:

• The amount of body fat changes with age. (BMI for children and teens is often referred to as BMI-for-age.)
• The amount of body fat differs between girls and boys.

The CDC BMI-for-age growth charts for girls and boys take into account these differences and allow translation of a BMI number into a percentile for a child’s or teen’s sex and age.

4. Find the weight status category for the calculated BMI-for-age percentile as shown in the following table. These categories are based on expert committee recommendations.

See the following example of how some sample BMI numbers would be interpreted for a 10-year-old boy.

The CDC BMI-for-age growth charts are available at: CDC Growth Charts: United States.

Is BMI interpreted the same way for children and teens as it is for adults?

Although the BMI number is calculated the same way for children and adults, the criteria used to interpret the meaning of the BMI number for children and teens are different from those used for adults. For children and teens, BMI age- and sex-specific percentiles are used for two reasons:

• The amount of body fat changes with age.
• The amount of body fat differs between girls and boys.

The CDC BMI-for-age growth charts take into account these differences and allow translation of a BMI number into a percentile for a child’s sex and age.

For adults, on the other hand, BMI is interpreted through categories that do not take into account sex or age.
 

Why can’t healthy weight ranges be provided for children and teens?

Healthy weight ranges cannot be provided for children and teens for the following reasons:

• Healthy weight ranges change with each month of age for each sex.
• Healthy weight ranges change as height increases.

What are the health consequences of overweight for children and teens?

Children and teens who are overweight may begin to experience health consequences during their youth as well as putting themselves at risk for weight-related health problems later in life.

Overweight children and teens have been found to have risk factors for cardiovascular disease, including high cholesterol, elevated insulin levels, and elevated blood pressure during childhood. One study showed that approximately 60% of overweight children had a least one cardiovascular risk factor, such as high cholesterol or high blood pressure; in comparison, only 10% of children with healthy weight had at least one risk factor. Additionally, 25% of overweight children had two or more risk factors.²

Other health consequences include the following potential problems:³

• Type 2 diabetes
• Sleep apnea (not breathing for at least 10 seconds during sleep)
• Social consequences including poor self-esteem and social discrimination

In addition to the health problems they may experience during their youth, overweight children and teens are at increased risk for various chronic diseases as adults (including hypertension, type 2 diabetes, and coronary heart disease). Overweight adolescents are at greater risk of becoming overweight or obese as adults,⁴ about one third of all severely obese adults were overweight children.⁵

Maintaining a healthy weight during childhood and adolescence may reduce the risk of becoming overweight or obese as an adult. Encourage children and teens to keep up healthy eating habits, participate in physical activity on most (preferably all) days of the week, and limit television viewing. For more information, visit Tips for Promoting Healthy Eating and Physical Activity for Children and Teens.

How can I tell if my child is overweight?

CDC and the American Academy of Pediatrics (AAP) recommend the use of Body Mass Index (BMI) to screen for overweight in children and teens aged 2 through 19 years. Although BMI is used to screen for overweight in children and teens, BMI is not a diagnostic tool.

For example, a child who is relatively heavy may have a high BMI for his or her age. To determine whether the child has excess fat, further assessment would be needed. Further assessment might include skinfold thickness measurements. To determine a counseling strategy, assessments of diet, health, and physical activity are needed.

Can I determine if my child or teen is overweight by using an adult BMI calculator?

No. The adult calculator provides only the BMI number and not the BMI age- and sex-specific percentile that is used to interpret BMI and determine the weight category for children and teens. It is not appropriate to use the BMI categories for adults to interpret BMI numbers for children and teens.

My two children have the same BMI values, but one is considered overweight and the other is not. Why is that?

The interpretation of BMI-for-age varies by age and sex so if the children are not exactly the same age and of the same sex, the BMI numbers have different meanings. Calculating BMI-for-age for children of different ages and sexes may yield the same numeric result, but that number will fall at a different percentile for each child for one or both of the following reasons:

• The normal BMI-related changes that take place as children age and as growth occurs.
• The normal BMI-related differences between sexes.

See the following graphic for an example for a 10-year-old boy and a 15-year-old boy who both have a BMI-for-age of 23. (Note that two children of different ages are plotted on the same growth chart to illustrate a point. Normally the measurement for only one child is plotted on a growth chart.)

References

¹Mei Z, Grummer-Strawn LM, Pietrobelli A, Goulding A, Goran MI, Dietz WH. Validity of body mass index compared with other body-composition screening indexes for the assessment of body fatness in children and adolescents. American Journal of Clinical Nutrition 2002;7597–985.

²Freedman DS, Dietz WH, Srinivasan SR, Berenson GS. The relation of overweight to cardiovascular risk factors among children and adolescents: The Bogalusa Heart Study. Pediatrics 1999;103:1175–1182.

³Must A and Anderson SE. Effects of obesity on morbidity in children and adolescents. Nutrition in Clinical Care 2003;6(1):4–12.

⁴Whitaker RC, Wright JA, Pepe MS, Seidel KD, Dietz WH. Predicting obesity in young adulthood from childhood and parental obesity. New England Journal of Medicine 1997;37(13):869–873.

⁵Ferraro KF, Thorpe RJ Jr, Wilkinson JA. The life course of severe obesity: does childhood overweight matter? Journal of Gerontology: Social Sciences 2003;58B(2):S110–S119.

 Information taken from cdc.gov