How to Calculate Your Child's BMI

Calculating Weight-for-Height Per Age and Sex

Body mass index (BMI) is an important measurement used to determine whether your child is overweight, underweight, or at an ideal weight for his or her age. So important is BMI in monitoring your child's health that the U.S. Preventive Services Task Force currently recommends the routine tracking of BMI in all children over the age of two.

How Child and Adult Calculations Differ

Unlike adults for whom a BMI of 25 to 30 is considered overweight and a BMI is classified as obese, children require a different standard of evaluation given that their bodies change so rapidly.

The calculation starts with a standard BMI based on your child's height and weight. BMI doesn't measure body fat but rather the volume of tissue (including muscle, fat, and bone) in your child's body. The value is then used to determine where your child falls, percentage-wise, among children of the same sex and age group.

That percentile value will be used to determine your child's weight category (underweight, normal, overweight, obese).

Getting the Right Measurements

father measuring son's height
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To get an accurate BMI calculation, you will need to ensure accurate height and weight measurements each and every time. To do so:

  • Remove your 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 the head, shoulders, back, buttocks, and heels placed against a flat wall surface. Be sure your child neither slouches or lifts the heels.
  • To get an accurate weight, use a digital scale placed on a level, uncarpeted floor. Have the child place both feet on the center of the scale without moving. Avoid spring-loaded bathroom scales which can wear out over time.

Always try to use the same tools for measurement and perform the measurements at the same time of day (such as in the morning just before breakfast or in the afternoon right before dinner).  

Calculating Your Child's BMI

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The standard formula to calculate BMI is weight ÷ (height x height) x 703. To calculate this with imperial (pounds and inches) measurements:

  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 703.

The metric formula to calculate BMI is weight ÷ (height x height) x 10,000. To calculate this:

  1. Measure the child's weight in kilograms (not grams). This may be expressed in a decimal value, such as 12.7 kilograms.
  2. Measure the child's height in centimeters (not meters). This would be expressed, for example, as 105 centimeters and not 1.05 meters.
  3. Calculate the BMI by dividing the weight by the height squared and multiplying that total by 10,000.

There are also a number of online BMI calculators that can simplify the process, including one offered by the National Heart, Lung, and Blood Institute. 

BMI Percentiles for Children

BMI percentile categories
CDC

The BMI percentile allows us to categorize your child's BMI based where he or she falls in comparison to other boys or girl of the same age group. The classification system was established in 1977 to categorize a child's development based on weight-for-height percentiles.

The value is expressed not as a percentage (such as 85%) but rather as a percentile (such as the 85th percentile of all children of that sex and age group).  If your child is, say, in the 75th percentile, that would mean that he or she is higher than 75 percent of children in the group.

In terms of weight, higher percentile values correspond to higher weight categories, while lower percentile values correspond to lower weight categories.

Your child's BMI percentile can be determined by plotting the BMI on a BMI chart.

Weight Categories for Children

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Once you have established your child's BMI and percentile, you can then see exactly where he or she falls within the expected range.

The BMI-for-age weight categories were based on recommendations from an expert committee organized by the American Medical Association (AMA), the Centers for Disease Control and Prevention (CDC), and the Human Resources Services Administration (HRSA).

The weight categories for children are defined as follows:

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

Sources:

Centers for Disease Control and Prevention. Using the CDC BMI-for-age Growth Charts to Assess Growth in the United States Among Children and Teens Aged 2 Years to 20 Years. Atlanta, Georgia; updated September 9, 2014.

U.S. Preventive Services Task Force. Screening for Obesity in Children and Adolescents: US Preventive Services Task Force Recommendation Statement. JAMA. 2017;317(23):2417-26. DOI: 10.1001/jama.2017.6803.