A child's body mass index (BMI) allows you to compare that child's weight to those of other children of the same age and gender. The BMI is a ratio of weight to height and can change rapidly in children since their height is still increasing. This means that a child's BMI percentile score is more meaningful than the raw BMI score.
Step 1
Weigh the child in pounds on a bathroom scale. Measure the child's height in inches with a tape measure.
Step 2
Compute the child's BMI. This is given by the equation B = W / (H x H) x 703 where B is the BMI, W is the child's weight and H is the child's height. Multiplication by 703 is necessary to convert the BMI from units of pounds per square inch to kilograms per square meter (kg/m^2.) Most BMI references use metric units.
Step 3
Locate the child's position on the BMI chart. Each gender uses a different BMI chart, so you need to select the correct chart for the child's gender. Draw an imaginary vertical line on the chart for the child's age and an imaginary horizontal line for the child's BMI. The child's position on the BMI chart will be the point on the chart where these two lines intersect.
Step 4
Determine the child's BMI percentile score. Compare the child's location on the BMI chart with the percentile curves. This will allow you to estimate the child's BMI percentile score.
Step 5
Interpret the child's BMI percentile score. A BMI percentile score below 5 percent indicates the child is underweight; a BMI percentile score between 85 and 95 percent indicates the child is at risk for being overweight; and a BMI percentile score greater than 95 percent indicates the child is overweight.
Things You'll Need
- Scale
- Tape measure
- Calculator
- BMI chart
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