Body mass index (BMI) is a ratio between a person's weight and height. A doctor frequently uses the BMI to determine whether a patient is at his recommended weight. In the case of children, however, this number is only meaningful when compared to the BMI of other children in the same age and gender group. To compare this information, you must convert the raw BMI score to a BMI percentile.
Step 1
Weigh your child in pounds on a bathroom scale. This example assumes your child weighs 103 lb.
Step 2
Measure your child's height in inches with a tape measure. This example assumes your child is 61 in. tall.
Step 3
Compute your child's BMI. Your child's BMI is given by the equation BMI = [weight / (height x height)]. Your child's weight is in pounds, and your child's height is in inches, so your child's BMI will be in units of pounds per square inch (lb./sq. in.).
This example assumes your child weighs 103 lb. and is 61 in. tall. Your child's BMI is therefore [103 / (61 x 61)] = 0.0277 lb./sq. in.
Step 4
Convert your child's BMI from units of pounds per square inch to units of kilograms per square meter (kg/sq. m). To make the conversion, multiply your child's BMI in pounds per square inch by 703.
This example assumes your child's BMI is 0.0277 lb./sq. in. Your child's BMI is therefore 0.0277 x 703 = 19.5 kg/sq. m.
Step 5
Determine your child's BMI percentile based on his age, gender and BMI. Locate your child's BMI on a BMI chart's vertical line and your child's age on the chart's horizontal line. Cross-reference these two values to estimate your child's BMI percentile.
This example assumes your child is a 13-year-old boy with a BMI of 19.5 kg/sq. m. His BMI percentile is approximately 63.
Things You'll Need
- Bathroom scale
- Tape measure
- Calculator
- BMI chart
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