Health care professionals use BMI-for-age, which charts height and weight against age to screen children and adolescents between the ages of two and 20 for underweight, overweight and obesity. BMI-for-age charts show age-related changes in growth and alert parents and doctors to early signs that a child is gaining weight too quickly or gaining too little weight over time. BMI-for-age correlates with clinical risk factors for chronic diseases, including high blood pressure, elevated insulin levels and hyperlipidemia, or too much fat circulating in the blood.
Step 1
Divide body weight in pounds by height in inches squared, and then multiply by a conversion factor of 703, according to the Centers for Disease Control's standard BMI equation: BMI = weight in lb. / (height in inches)2 x 703. For example, if the child or adolescent is 85 lb. and 5 feet tall, the calculation looks like this: 85 lb. divided by 60 inches squared, or 3,600 inches, and then multiplied by 703, or 85/3600 X 703 = 16.6.
Step 2
Find the BMI to the nearest tenth on both vertical BMI scales, on the left- and right-hand sides of the growth chart. Mark the scales, and draw a horizontal line with a ruler to connect the two marks.
Step 3
Find the age to the nearest quarter of a year on the horizontal scale at the bottom of the growth chart. Draw a vertical line upward to the top of the chart to create a line for your age.
Step 4
Locate the intersection of the two lines, which represents the BMI-for-age. Darken the intersection with a dot so that it will be easily noticeable.
Step 5
Look at the J-shaped curves that identify the fifth, 10th, 25th, 50th, 75th, 90th and 95th percentiles. The BMI-for-age falls between or on one of these lines, which identifies the BMI percentile. For example, if the child is 13 years old and has a BMI of 16.6, the BMI-for-age percentile falls between the 10th and 25th percentiles.
Tips and Warnings
- Use the appropriate chart for a child: Growth charts for children are gender- and age-specific. Use the same BMI-for-age percentile chart over time. The Children's Nutrition Research Center advises using the same chart to plot BMI-for-age every six months. Leave the dots intact even if you erase the horizontal and vertical lines so you can track the changes over time. If a dot falls between two J-shaped percentile lines, cite both when stating the percentile, as in the statement, "My BMI-for-age falls between the 80th and 85th percentiles."
- Consult a doctor or other health professional if you feel the need for further assessments.
Things You'll Need
- Calculator
- Ruler
- BMI-for-age growth chart
References
- Centers for Disease Control: About BMI for Adults
- Baylor College of Medicine: BMI Graph Page
- Centers for Disease Control: Using the BMI-For-Age Growth Charts
- Centers for Disease Control: Use and Interpretation of the CDC Growth Charts
- "National Health Statistics Reports": Changes in Terminology for Childhood Overweight and Obesity; C. L. Ogden, K. M. Flegal; June 25, 2010
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