Body Mass Index for children and teens (2-20 years)

AUGUST 16, 2021

Body mass index (BMI) is used as a screening tool to identify weight categories that may lead to possible health problems in children. It is an important aspect of physical assessment and is a reliable indicator of body fatness for most people. For children and teens, BMI is age and sex-specific and is often referred to as BMI for age. It determines whether a child is in a healthy weight category for his height, age, and gender. Being overweight or underweight put children at health risk. BMI is interpreted differently for children and teens even though calculated with the same formula as for calculating BMI for adults aged 18-65 years. Due to changes in height and weight with age as well as their relation to body fats BMI levels among children and teens are expressed relative to other children of the same age and sex in a reference population.

The Metric formula to calculate BMI is:

  • Measure the child’s weight in kilograms.
  • Measure the child’s height in centimeters and change it to meters by dividing it by 100.
  • Calculate the BMI by dividing the weight by the height squared.

When using Imperial measurements :

  • Measure the child’s weight in pounds.
  • Measure the child’s height in inches.
  • Calculate the BMI by dividing the weight by the height squared and multiplying the total by a conversion factor, 703 to convert from lbs. / inches2 to kg/m2.

After BMI is calculated it is expressed as a percentile obtained from either a graph or a percentile calculator. These percentiles express a child’s BMI relative to other children of the same age and sex.

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A child with BMI of 95 percentile means his BMI is greater than 95% of 10 year old boys in the referring population.

The following is an example of how sample BMI numbers will be interpreted for a 10-year-old boy:

Body mass index-for-age percentiles of Boys from age 2-20 years (CDC Growth Chart)

Once you have established your child’s BMI and percentile, you can then see exactly where he or she falls within the expected range.

Percentile RangeWeight Status Category
less than 5 percentile.Underweight
5 percentile to less than 85 percentile.Healthy
85 to less than 95 percentile. Overweight
Equal to or greater than 95 percentile.Obese

BMI does not directly measure body fat but correlates with more direct measures of body fat. High BMI can be due to extra muscles but to determine if excess fat is a problem a health care provider will perform further investigation. The BMI for age percentile growth charts is the most commonly used indicator to measure the size and growth patterns of children and teens. Adjustments for age and sex are crucial as BMI varies widely with age throughout childhood and adolescence, especially during puberty. However, there is a general consensus that ‘overweight’ starts at the 85th percentile of age-specific BMI growth charts, while obesity is defined as at or above the 95th percentile.

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Although analysis of information about an individual’s growth can be complex, clinicians often look for patterns that may indicate abnormal growth. Diagnostic accuracy when screening for overweight and obesity in specific populations can be improved with the use of nationally specific BMI-for-age charts. Adiposity is highly variable and dependent on ethnic groups. Height and weight curves may not be optimal fits in all cases. The importance of growth monitoring in pediatric care is well recognized. Unduly slow or rapid growth can indicate serious medical conditions, including genetic disorders, chronic disease, infectious disease, abuse or neglect, and a variety of other problems.

Adverse effects of obesity

Childhood obesity can adversely affect nearly every organ system and often causes serious consequences, including hypertension, dyslipidemia, insulin resistance, dysglycemia, fatty liver disease, and psychosocial complications. It is important to prevent childhood obesity as well as to identify overweight and obese children at an early stage so they can begin treatment and attain and maintain a healthy weight.

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