How to Determine Child BMI
According to the Centers for Disease Control and Prevention, a child's body mass index, or BMI, is a reliable indicator of the fat content contained in most children and teens. Knowing a child's BMI can help parents and doctors screen for potential health problems later in life, such as obesity or diabetes. The recommended BMI for a child will vary based on the child's age and gender. Accurately measuring the child's height and weight can lead to a reliable reading of BMI.
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Instructions
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Measuring Height
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Remove your child's shoes, hair ornaments or hair designs, which could interfere with a height measurement.
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Instruct to the child to stand with his feet flat and together, up against a wall. The child's legs should be straight and his arms should hang at his sides so that his shoulders are even.
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Turn the child's head so that he or she is looking straight ahead, so that his line of sight is parallel with the floor. Instruct him not to look up or down. Check that his head, shoulders and heels are all touching the wall, or as close as they can get, depending on your child's body shape.
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Kneel in front of your child (or stand, if he or she is tall) so that your eyes are at the same level with the crown of the child's head. Use a flat headpiece (like a ruler or yardstick) to form a right angle with the wall.
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Lower the headpiece until it rests on the crown of the child's head. Make a light mark on the wall with a pencil where the bottom of the headpiece touches the wall.
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Allow the child to move away from the wall. Use a tape measure to accurately measure the distance from the floor to the mark on the wall. Measure within the nearest 1/8 inch. Record the measurement.
Measuring Weight and Calculating BMI
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Place a digital scale flat on firm flooring (wood or tile, not carpet). Have your child remove his or her shoes and all heavy clothing, such as sweaters or bulky pants. Make sure he has nothing in his pockets.
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Instruct the child to stand with both feet as close to the center of the scale as he can. Wait until the scale stops moving; record the weight to the nearest decimal.
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Access the website for the Centers for Disease Control and Prevention. The site features a calculator for BMI in children; enter your child's birth date, the date you took the measurement, your child's sex, and the height and weight measurements. If you cannot access the site, move to step four.
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Calculate your child's BMI with the CDC's formula: Take the child's height in inches and square the number, or multiply it by itself. When you have the squared number, divide the weight in pounds by that number. Finally, multiple the number by 703. For example, say your child is 48 inches and 110 pounds. Square 48 to get 2,304. Divide the weight by the new number: 110 divided by 2,304 is .04774. Multiply that number by 703 to get 33.6. This is your child's percentile number.
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Compare your child's BMI measurement to the Center for Disease Control and Prevention's growth charts to determine his health. In general, a child is underweight if he falls under the 5th percentile on the chart; he is healthy if he measures between the 5th percentile and the 85th percentile; he is overweight if he measures between the 85th and 95th percentile, and he is obese if he falls at or above the 95th percentile.
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Tips & Warnings
Measure a child's height on a flat floor that has no carpeting, and use a wall with no molding; these extra supports can interfere with the measurement.
For metric units, take your child's weight in kilograms and height in meters. To calculate BMI with the CDC's formula, square the height measurement. Divide the weight by the height squared to get your child's percentile.
Calculating your child's BMI will not accurately diagnose any problems without further work. Consult your physician or pediatrician if you fear your child is obese or has a medical problem.
References
- Photo Credit scale image by PinkShot from Fotolia.com
