How to Calculate Child's BMI

Body Mass Index (BMI) is a method of measuring the proportion of height and weight to age and determining where that percentage falls on a scale of being considered overweight, normal or underweight. The BMI is not an indicator of the percentage of body fat or muscle in a child and should be used only as part of the overall evaluation of a child's muscle or fat percentage. The smallest error in measuring height or weight can dramatically change the results on the graph, so be careful and patient when performing this calculation.

Things You'll Need

• Tape measure
• Weight scale
• Calculator
• Child BMI graph

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Instructions

• 1

  Measure your child's height in inches. Then do it again. Keep measuring your child until you can do it three times in a row and come up with the identical measurement. The slightest error in height will throw off the BMI results significantly.

• 2

  Weigh your child. Do this while your child is in his or her underwear. Any additional clothing weight will throw off the BMI results.

• 3

  Multiply your child's height by itself and then write down that result. Next, divide the weight of your child by the answer you got in the first part. Finally, multiply this by 703. The answer you get is your child's Body Mass Index. The full equation looks like this: BMI = [weight / (height x height)] x 703.

• 4

  Look up your child's BMI on the appropriate gender chart listed in the resources below. The chart will tell you the range your child falls into (overweight, underweight or normal) for his or her age and sex.