How to Calculate Infant BMI

Father feeding his baby boy
You can interpret a child's BMI by plotting it on a growth chart. (Image: Purestock/Purestock/Getty Images)

Body Mass Index, or BMI, measures the relationship between a person’s height and weight. Your BMI indicates whether you are underweight, overweight, obese or at a healthy weight. You can use the same formula to calculate an infant’s BMI that you would use to calculate an adult’s BMI. You must interpret infant BMI values differently than you would adult BMI values, however. In young children, a healthy relationship between weight and height also depends on age and gender. The Centers for Disease Control and Prevention recommend using growth charts to interpret infant BMI.

Step 1

Weigh the child and record the weight in pounds.

Step 2

Measure the child’s height or length and record the measurement in inches.

Step 3

Divide the child's weight in pounds by his height in inches, and then divide the resulting number once again by the child's height in inches.

Step 4

Multiply the answer in Step 3 by 703.

Step 5

Plot this BMI value on a growth chart for the appropriate age. Locate the child’s age on the horizontal axis and her BMI on the vertical axis. Use a ruler to draw straight vertical and horizontal lines from these points and draw a dot where they intersect on the growth chart. The zone where this dot falls on the growth chart should indicate the percentile rank of the child’s BMI.

Step 6

Interpret the BMI percentile rank. For example, a BMI plotted on the appropriate age chart with a percentile rank greater than 95 indicates that the child is overweight, and a BMI with a percentile rank between 85 and 95 indicates that the child is at risk of being overweight, according to the CDC. Consult a pediatrician for help interpreting your child’s BMI and the BMI’s percentile rank.

Tip

Measure your child’s BMI frequently and plot each new measurement on the growth chart. The Centers for Disease Control and Prevention publish growth charts designed for this purpose. Ask your child’s physician for more information about these charts.

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