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How to Calculate BMI Percentiles

Health-care professionals use the body mass index, or BMI, as a simple way to assess the amount of a person’s body fat. Unlike direct fat measurements, such as the caliper test or underwater weighing, BMI uses a height and weight calculation to come up with a BMI score. Adults use their raw BMI scores to find their classification on a standard BMI chart. Because children and teenagers are still growing, their BMI scores are plotted on gender-specific age-growth charts that compare them with other children of the same age and gender across the nation. These percentiles are then categorized as underweight, healthy weight, overweight or obese. BMI percentiles only apply up to the age of 20.

Step 1

Weigh your child in pounds on an accurate bathroom scale. If you have a digital scale, use the exact reading for the BMI calculation—don’t round the figure up or down.

Step 2

Measure your child's height in inches. Have him stand against a wall with his shoes off and use a pencil to lightly mark the wall where the crown of his head hits it.

Step 3

Calculate your child's BMI score by dividing her weight by her squared height, and multiplying the result by 703, which is a conversion factor. As an example, if she weighs 110 pounds and is 60 inches tall, the BMI score would be 21.5 [weight ÷ (height in inches x height in inches )] x 703 = BMI score.

Step 4

Use the appropriate age-growth chart to find your child's BMI percentile relative to other children or teenagers of the same gender and age. For example, if your child is a 17-year-old girl with a BMI score of 21.5, she'd be in the 50th percentile for your age and gender, which is considered a healthy weight. However, a 10-year-old girl with the same BMI score would be just above the 85th percentile, and considered overweight.

Items you will need

  • Bathroom scale
  • Tape measure
  • BMI chart

Tip

  • Children or teenagers whose BMI score falls at or below the 5th percentile for their gender and age are considered underweight. A healthy weight is characterized by landing between the 5th and the 85th percentiles. Overweight children and teens have BMI scores between the 85th and 95th percentiles, while it’s considered obese to be above the 95th percentile.

Photo Credits:

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This article reflects the views of the writer and does not necessarily reflect the views of Jillian Michaels or JillianMichaels.com.