How to Calculate BMI for Children

Are there children in your family you think are either overweight or underweight? Weight problems can lead to other health problems in children, so it's important to determine whether a child really does have a weight problem. A good indication is the child's "body mass index."BMI (also called Quetelet Index) is designed to identify a percentage of either extra or diminished weight in a person considering his/her height. This article will tell you how to calculate the BMI of a child. Review the steps and warnings below, and remember that the calculation of an adult's BMI can sometimes differ from that of a child. The calculation can be used for both children and teenagers according to the table of BMI, which takes age and gender into consideration.(That's not as important for adults.) You should calculate BMI percentile for children up to 20 years of age. BMI percentile calculation can help adults detect weight problems in kids. The procedure will take about ten minutes and is not difficult at all.


  1. Image titled Calculate BMI for Children Step 1
    Weigh the child on a suitable scale. Write down the results.
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    Determine the height of the child. Touch one end of the tape measure to the floor and unroll it to the top of the head. Another method is to stand the child against a wall and mark the top of the head on the wall with a pencil. Then measure the distance from the floor to the mark. Record the figures on the sheet of paper.
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    Calculate BMI according to this formula: BMI is weight (in kilograms) divided by the square of the height (in meters). If you measure your child in inches and pounds, you will multiply the weight in pounds by 703 and divide the result by the square of the height in inches. For example, here is the calculation of BMI for a boy 49 inches (124.5 cm) tall and weighing 52 pounds. You would multiply 703 by 52 and divide that by the square of 49. Thus, 52 times 703 divided by 2401 equals 15.225, which is the boy's body mass index. For a metric example, a girl weighs 29 kilograms and stands 1.29 meters (4.2 ft) tall. Her BMI is 29 divided by the square of 1.29. Thus, 29 divided by 1.66 equals 17.47, her BMI. (For easy conversion between metric and standard measurements, see the Tip below.)
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    Find a Body Mass Index chart online. See where the child's height intersects with his/her weight, and the chart will indicate where the child is on the "overweight/underweight" spectrum, taking age and gender into account. In our example above, a 14-year-old girl having a BMI of 17.46 is at a very low percentile for her age. At or below the fifth percentile, she is considered underweight and a candidate for health problems.
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    Understand the significance of percentiles.
    • A child at or below the fifth percentile is considered underweight.
    • A child between the fifth and 85th percentile is considered to be of healthy weight.
    • A child between the 85th and 95th percentile is considered borderline overweight and potentially at risk for health problems as a result.
    • A child in the 95th percentile or above is considered overweight and could expect health problems.


  • Kilograms equal pounds divided by 2.2.
  • Here's how to convert to or from metric measurement:
  • Meters equal inches multiplied by .0254.
  • Inches equal meters divided by .0254 (or multiplied by 39.37).
  • Pounds equal kilograms multiplied by 2.2.


  • When measuring the child's height, be sure s/he is standing up straight.
  • It is better to weigh the child in the morning before breakfast.
  • Each time you weigh the child, have him or her wear as little as possible to make the measurement more accurate and meaningful.
  • Be sure your scale is accurate.

Things You'll Need

  • A pen or pencil and paper
  • Weight scale
  • Tape measure or gauge on the wall
  • A calculator
  • A BMI chart showing percentiles

Article Info

Categories: Childhood Health