A Guide to Calculating Your Kid's BMI

Updated December 01, 2017

Your child's body mass index (BMI) is an important tool used to determine if your child is overweight, underweight, or at a healthy weight. The American Academy of Pediatrics today recommends routine tracking of BMI in all children once they are around the age of two.

BMI is calculated with a child's height and weight using either a simple formula. It doesn't measure body fat but rather the amount of tissue mass (muscle, fat, and bone) in the child. This value is then used to categorize the child as being as either underweight, normal weight, overweight, or obese.

It is relatively easy to interpret results in adults as a BMI between 25 to 30 is considered overweight and a BMI of 30 or above is considered obese.

It is a bit more complicated with children since you also have to consider the child's age and sex to determine the percentile range he or she may fall within. The BMI percentile is then used to categorize the child's weight based on the averages seen within that group.

Getting the Right Calculations

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Getting the right BMI calculation requires you to get the accurate height and weight measurements. To do so:

  • Remove the child's shoes, outer clothing, and hair ornaments.
  • To get an accurate height, have the child stand with his or her feet together on an uncarpeted floor with head, shoulders, back, buttocks, and heels placed against a flat wall surface.
  • To get an accurate weight, use a digital scale placed on a level, uncarpeted floor and have the child place both feet on the center of the scale without moving. Avoid spring-loaded bathroom scale.

Calculating BMI in Children

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The standard formula to calculate a child's BMI is weight / (height x height) x 703. To calculate this, you would need to:

  1. Measure the child's weight (in pounds).
  2. Measure the child's height (in inches).
  3. Calculate the BMI by dividing the weight by the height squared and multiplying that total by a conversion factor of 703

The metric formula to calculate a child's BMI is weight​ / (height x height). To calculate this, you would need to:

  1. Measure the child's weight (in kilograms).
  2. Measure the child's height (in meters).
  3. Calculate the BMI by dividing the weight by the height squared.

BMI Percentiles for Children

BMI percentile categories
Centers for Disease Control and Prevention

The BMI percentile allows us to categorize a child BMI based on other boys or girl in his or her age group. In order to determine the correct percentile, you would need to:

  1. Find the age and sex appropriate for your child on a BMI chart.
  2. Plot your child's BMI on the chart.

Weight Categories for Children

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Once you have established your child's BMI and percentile, you will then see exactly where he or falls within the expected range. The weight categories are defined as follows:

  • Underweight is categorized as having a BMI less than the 5th percentile.
  • Normal weight is categorized as having a  BMI from the 5th percentile to below the 85th percentile
  • Overweight is categorized as having a BMI above the 85th percentile to below the 95th percentile
  • Obese is categorized as having a BMI greater than or equal to the 95th percentile.