A child's body mass index, or BMI, is based on the child's height and weight. It's a way of comparing that child's physical development with that of other children with the same age and gender. A child's BMI changes rapidly, so the raw BMI score isn't as important as the BMI percentile score. You can obtain the BMI percentile score from the raw BMI score by using a BMI chart.
Step 1
Use a scale to weigh the child and a tape measure to determine the child's height.
Step 2
Compute the child's raw BMI score with the equation BMI = 703 x w / h^2. This will provide the raw BMI score where w is the child's weight in pounds and h is the child's height in inches. The conversion factor of 703 will convert the BMI from pounds per square inch into kilograms per square meter (kg/m^2.) This conversion is necessary because most BMI charts use BMI values in metric units.
Step 3
Calculate a sample BMI score. For example, assume your child weighs 87 pounds and is 60 inches tall. The raw BMI score is 703 x 87/60^2 = 703 x 87/3600 = 17.0 kg/m^2.
Step 4
Derive the child's BMI percentile score from the raw BMI score. Use the correct BMI chart for your child's gender and locate your child's age on the horizontal axis of the chart. Locate your child's raw BMI score on the vertical axis of the chart. Locate the point where these two lines intersect to identify the BMI percentile score.
Step 5
Determine a sample BMI percentile score. Assume your child is a 10.5-year-old boy with a raw BMI of 17.0 k/m^2. The vertical line on the boy's chart for 10.5 years and the horizontal line on the boy's chart for a BMI of 17.0 k/m^2 intersect approximately on the 50th percentile curve.
Things You'll Need
- Scales
- Tape measure
- Calculator
- BMI chart
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