The body mass index (BMI) is a method of comparing body weight to height. It's commonly used to classify a child's weight relative to that of other children of the same age and sex. The BMI percentile in children is therefore more meaningful than the raw BMI score. The BMI is used in conjunction with other factors to determine if a child is overweight. You can calculate your child's BMI, then compare it to standard charts to get an idea of how healthy your child's weight is.
Step 1
Compute the child's raw BMI score directly from measurements. Divide the child's weight kilograms by the square of his height in meters. Most BMI charts use BMI values in metric units of kilograms per square meter (k/m^2).
Step 2
Convert a BMI in English units of measure to metric units. You will commonly measure weight in pounds and height in feet or inches, which will require you to convert the BMI to kilograms and meters. Convert a BMI score in units of pounds per square inch (lb/in^2) by multiplying by 703. Convert a BMI score in units of pounds per square feet (lb/ft^2) by multiplying by 4.88.
Step 3
Calculate a specific BMI score. For example, assume your child weighs 87 lbs. and is 5 feet tall. The raw BMI score is 87/5^2 = 87/25 = 3.48 lb/ft^2. Convert this BMI to metric units by multiplying 3.48 by the conversion factor of 4.88 provided in Step 2. The metric BMI is therefore 3.48 x 4.88 = 16.9 k/m^2.
Step 4
Determine the BMI percentile score for your child. Select the appropriate BMI chart for your child's gender and cross reference your child's age on the horizontal line with the BMI score on the vertical line (see Resources). Compare this point on the chart with the percentile curves to establish your child's BMI percentile score. For example, assume your child is a boy and is 10 ½ years old. A BMI of 16.9 k/m^2 and an age of 10 ½ years intersect at the 50th percentile curve. This child therefore has an average BMI for his age and sex.
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