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For information about the consequences of childhood obesity, its contributing factors and more, see Tips for Parents Ideas and Tips to Help Prevent Childhood Obesity.
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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.
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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, overweight, and obese).
BMI-for-age weight status categories and the corresponding percentiles are shown in the following table.
| Weight Status Category | Percentile Range |
|---|---|
| Underweight | Less than the 5th percentile |
| Healthy weight | 5th percentile to less than the 85th percentile |
| Overweight | 85th to less than the 95th percentile |
| Obese | Equal to or greater than the 95th percentile |
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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 and obesity in children beginning at 2 years old.
For children, BMI is used to screen for obesity, overweight, healthy weight, 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.
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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. See Measuring Children's Height and Weight Accurately At Home.
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.
| Weight Status Category | Percentile Range |
|---|---|
| Underweight | Less than the 5th percentile |
| Healthy weight | 5th percentile to less than the 85th percentile |
| Overweight | 85th to less than the 95th percentile |
| Obese | Equal to or greater than the 95th percentile |
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.
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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.
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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.
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How can I tell if my child is overweight or obese?
CDC and the American Academy of Pediatrics (AAP) recommend the use of Body Mass Index (BMI) to screen for overweight and obesity in children and teens aged 2 through 19 years. Although BMI is used to screen for overweight and obesity 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.
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Can I determine if my child or teen is obese 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.
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My two children have the same BMI values, but one is considered obese 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.)
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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;7597985.
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;103:11751182.
3Must A and Anderson SE. Effects of obesity on morbidity in children and adolescents. Nutrition in Clinical Care 2003;6(1):412.
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):869873.
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):S110S119.
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