A child’s body mass index, or BMI, is a calculation that compares the child’s height and weight. Health care workers commonly compare a child’s BMI to those of other children with the same age and gender to obtain a BMI percentile score. You can then calculate the normal range for a child’s weight from specific BMI percentile values. These values are commonly available from standard sources such as the Baylor College of Medicine.
Step 1
Establish the normal BMI percentile range them for children. The normal BMI percentile range for children is between the 5th and 85th percentile, according to The Centers for Disease Control and Prevention.
Step 2
Locate the point on the BMI percentile chart where the 5th and 85th percentile curves intersect the vertical line for your child’s age. Record the corresponding raw BMI values.
Assume your child is a 16-year-old girl for this example. The 5th percentile corresponds to a raw BMI value of 16.8 kg/m^2. The 85th percentile corresponds to a raw BMI value of 24.6 kg/m^2.
Step 3
Convert the BMI values in step two from metric units to English units of measure by dividing them by 703.
A BMI value of 16.8 kg/m^2 is equal to a BMI of 16.8 / 703 = 0.0239 lb/in^2. A BMI value of 24.6 kg/m^2 is equal to a BMI of 24.6 / 703 = 0.0350 lb/in^2.
Step 4
Derive an equation that expresses weight in terms of BMI and height. BMI = Weight / (Height x Height) by definition, so Weight = BMI x Height x Height.
Assume your child is 62 inches tall for this example. Weight = BMI x Height x Height, so Weight = BMI x 62 x 62 for your child.
Step 5
Substitute the specific BMI values from step three into the specific equation for weight from step four. This will provide a specific normal weight range for your child.
The normal BMI range for your child is between one 0.0239 lb/in^2 and 0.0350 lb/in^2. Your child’s weight is equal to BMI x 62 x 62, so the normal range for your child’s weight is from [0.0239 x 62 x 62] lb. to [0.0350 x 62 x 62] lb. This is equal to a range of 92 to 135 lb.
Things You'll Need
- Calculator
- BMI chart
- Tape measure
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