Body mass index, or BMI, is used for adults, teenagers and children to compare weight and height. BMI is an inexpensive, easy and reliable indicator of body fatness for most children and teenagers. The Centers for Disease Control and Prevention and the American Academy of Pediatrics recommend the use of BMI to screen for overweight and obesity in children beginning at two years old. For children and teenagers, BMI is age- and sex-specific and is often referred to as BMI-for-age. Simple steps can be taken to calculate BMI for teenagers.
Step 1
Obtain accurate height and weight measurements. Measure the height of the teenager with shoes removed. Instruct her to stand up straight, take a deep breath and hold it, and look straight ahead. Record her height in inches.
Weigh the teenager with her shoes and as much other clothing removed as possible. Record her weight in pounds.
Step 2
Convert measurements. Convert inches to centimeters: 1 in. = 2.54 cm. Next, convert centimeters to meters: 1 m = 100 cm. For example, 72 in. x 2.54 = 182.9 cm/100 = 1.83 m. Convert pounds to kilograms: 1 kg = 2.2 lbs. For example, 150 lbs./2.2 = 68 kg.
Step 3
Calculate body mass index. Divide the weight of the teenager in kilograms by the height squared of the teenager in meters: BMI = WT(kg)/HT(m^2).
For example, if the teenager weighs 68 kg and is 1.83 m tall, the calculation would be: BMI = 68kg/(1.83m x 1.83m); BMI = 68kg/3.3489m; BMI = 20.3.
Record the teenager's BMI to one decimal place.
Step 4
Determine teenager's BMI percentile. Refer to the correct BMI percentile chart for the teenager's gender. Locate the teenager's age on the horizontal axis of the chart to the nearest year. Locate the teenager's BMI on the vertical axis of the chart. Record the point where the lines of the teenager's age and BMI intersect. Compare the point to the percentile curves to determine what percentile the teenager's BMI falls under. BMI between the 5th and 85th percentiles are considered to be within normal range for a child or teenager. Below the 5th percentile is considered underweight. BMI above the 85th percentile is considered overweight, while above the 95th percentile is considered obese.
Things You'll Need
- Scale
- Measuring tape
- Calculator
- BMI percentile chart
References
- Centers for Disease Control and Prevention: About BMI for Children and Teens
- "ACSM's Guidelines for Exercise Testing and Prescription," 7th Edition; Lawrence E. Armstrong, Ph.D., F.A.C.S.M., et al.; 2006
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