Body Mass Index Formula

Although the BMI number is calculated using the same body mass index formula 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.

Results of the body mass index formula for adults are interpreted through categories that do not take into account sex or age.

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.

CDC and the American Academy of Pediatrics (AAP) recommend the use of the Body Mass Index Formula 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.

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.

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.

The interpretation of BMI-for-age varies by age and sex so if two children are not exactly the same age and of the same sex, the BMI numbers have different meanings. Using the Body Mass Index Formula and 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.

Source: Centers for Disease Control and Prevention

Using Body Mass Index with Children and Teens?

 

Body Mass Index is a screening tool that is used to identify children with possible weight problems. 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.

BMI is not a diagnostic tool but is used to screen children for obesity, overweight, healthy weight, or underweight. For example, a child may have a high BMI for age and sex, but in order to determine if excess fat is a problem, a health care professional 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 Body Mass Index 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 Body Mass Index 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 so the Body Mass Index 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 Body Mass Index 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.

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.

Source: Centers for Disease Control and Prevention

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What is Body mass Index?

Body Mass Index (also known as 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 is not an exact measurment of body fat, but research has shown that it does correlate to direct measures of body fat. BMI can be considered an alternative for direct measures of body fat. BMI is also 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, overweight, and obese).

BMI-for-age weight status categories and the corresponding percentiles are shown below.

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

Source: Centers for Disease Control and Prevention

Preventing Obesity with Good Nutrition and Physical Activity

Unhealthy diet and the lack of physical activity can contribute to or aggravate many chronic diseases and conditions, including obesity, type 2 diabetes, hypertension, heart disease, stroke, and even some cancers.

During the past 20 years or so, obesity rates among adults have risen substantially in the United States. The 2005–2006, data from the National Center for Health Statistics shows us that 34% of over 72 million U.S. adults 20 years of age and older suffer from obesity.

In 2005, few adults met the Healthy People 2010 objectives for fruit and vegetable consumption with only 33% eating fruit two or more times each day and even fewer (27%) eating vegetables three or more times each day.

Despite the proven benefits that physical activity offers, less than half of American adults in 2007 engaged in enough physical activity to provide the needed health benefits.

More than a quarter of children born in 2004 were never breastfed.

The percentage of young people who suffer from obesity has almost tripled since 1980. During the 2003–2006 time frame, 16.3% of children and adolescents between the ages of 2–19 had a body mass index greater than or equal to the 95th percentile for age and sex on the CDC growth charts.

Over one third (34%) of adolescents in grades 9–12 had a non diet soft drink at least one time per day during the previous 7 day period.

A full 65% of young people in grades 9–12 did not get the recommended amount of physical activity in 2007 with 35% of them watching 3 or more hours of television on the average school day.

Source: Centers for Disease Control and Prevention

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