How to Calculate Relative Weight

Relative weight is calculated using the BMI and age of the individual.
Relative weight is calculated using the BMI and age of the individual. (Image: scale image by PinkShot from

Relative weight is a common measurement in pediatrics used to assess a child’s body mass index (BMI) relative to other children of the same age and sex. The BMI of children changes dramatically during the early years of life, thus necessitating the consideration of the child’s age when calculating their weight status category. Ranging from underweight to obese, relative weight categories are determined by which percentile the child’s BMI falls into compared to other children of the same age and sex.

Things You'll Need

  • Scale
  • Measuring Tape
  • Calculator

Calculating BMI

Measure the child’s weight on a bathroom scale. For the most accurate results, the child should not be wearing shoes or heavy clothing.

Determine the child's height using a strip of measuring tape or a ruler. The child should be standing straight up with her feet together, without shoes on.

Calculate the child’s BMI by plugging in the values you obtained in steps 1 and 2 into the appropriate formula. If your measurements are in pounds and inches, use the this formula: BMI = [(lbs) x 4.88]/(inches)^2. If your measurements are in kilograms and meters, use this: BMI = (kg)/(m)^2.

Determining Relative Weight Category

Locate the CDC BMI-for-age growth chart that contains the child’s exact age (see Resources). The CDC provides multiple growth charts to cover a wide range from birth to 20 years of age.

Pinpoint the intersection of the child's BMI and age on the growth chart. BMI will be located on the vertical axis and age on the horizontal axis. The point where these two values meet will determine the percentile for the child’s BMI-for-age.

Determine the child’s relative weight category. If the child’s BMI-for-age is less than the 5th percentile, she is considered underweight. A BMI-for-age between the 5th and 85th percentile is considered a healthy relative weight. Overweight individuals are those with a BMI-for-age between the 85th and 95th percentile. Anything above the 95th percentile is considered obese.

Tips & Warnings

  • This article should be used as a guide for determining relative weight. It is not meant to serve as medical advice. If you have concerns about your child's weight, talk to her physician.

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