About BMI for Children and Teens
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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.
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.
| Weight Status
Category |
Percentile
Range |
| Underweight |
Less than the
5th percentile |
| Healthy weight |
5th percentile
to less than the 85th
percentile |
| At risk of
overweight |
85th to less
than the 95th percentile |
| Overweight |
Equal to or greater than
the 95th percentile |
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.
| Weight Status
Category |
Percentile
Range |
| Underweight |
Less than the
5th percentile |
| Healthy weight |
5th percentile
to less than the 85th
percentile |
| At risk of
overweight |
85th to less
than the 95th percentile |
| Overweight |
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.
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.
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.
Content Source: Division of
Nutrition and Physical Activity, National Center for Chronic
Disease Prevention and Health Promotion
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