How to Calculate BMI in a 3 Year Old

The Body Mass Index (BMI) is the standard used by the World Health Organization to classify adults and children as underweight, overweight and obese. (Ref. 2.) BMI is used to identify weight problems in ages two to 19. For children and teens, BMI is age- and sex-specific and is called BMI-for-age, which differs from BMI for adults. According to the Centers for Disease Control and Prevention (CDC), age and sex are considered because the amount of body fat changes with age, and the amount of body fat differs between boys and girls. (Ref. 1.)

Things You'll Need

• Scale
• Tape measure or ruler

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Instructions

• 1

  Measure the child's height. Have the child stand up straight against a wall with no shoes on. Measure from head to toe, to the nearest 1/8 inch.

• 2

  Weigh the child to the nearest 1/4 pound. It's important to measure height and weight on the same day, as children grow rapidly.

• 3

  Calculate the three-year-old's BMI using the CDC's website. Enter the child's birth date. Enter the date of measurement. Select the sex of the child. Enter his or her exact height, in feet and inches. Enter his or her weight in pounds. Click "Calculate." This number is the child's BMI. (Res. 1.)

• 4

  Look at a child growth chart. This will determine the child's BMI-for-age percentile, which is how the BMI for children and teens is interpreted. There are growth charts on the CDC's website that display BMI-for-age percentile separately for boys and girls.

• 5

  Determine the child's weight status category. Consider the following:

  • Less than the 5th percentile is underweight.
  • 5th percentile to less than 85th percentile is healthy.
  • 85th to less than 95th percentile is overweight.
  • Equal to or greater than the 95th percentile is obese. (Ref. 1.)

  For example, if you look at the growth chart for boys age two to 20, you will see that a three-year old boy with a BMI of 16 is at the 50th percentile, so he is at a healthy weight.