BMI Calculator

BMI Calculator
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What is BMI?

Body Mass Index (BMI) is a measurement of a person’s weight with respect to his or her height. It is more of an indicator than a direct measurement of a person’s total body fat.

BMI, more often than not, correlates with total body fat. This means that as the BMI score increases so does a person’s total body fat.Body mass index is a calculation of child’s weight in relation to her height used to assess body fat and potential weight problems. This number alone is not diagnostic for children. Comparing BMI to other children of the same sex and age in growth charts helps to present a picture of your child’s weight. To get the best picture of a child’s growth, her growth needs to be charted for some time. A smooth growth curve as a child age reflects healthy growth.


Calculating Your Child's BMI

To determine your child’s BMI you need to divide her weight in pounds by her height in inches. Then divide this number by her height in inches again. Finally, multiply this number by 703. The answer reflects your child’s BMI. The CDC recommends that after completing the BMI calculation, compare it to a BMI percentile chart based on your child’s age and sex. A healthy BMI percentile lies between the 5th percentile and the 85th percentile. Anything below this classifies your child as underweight. Percentiles over this classify your child as overweight or obese. If you find your child’s BMI percentile does not fall in the healthy range, contact your child’s doctor for further evaluation.

Understanding the Numbers

BMI numbers have different meanings for children of different ages and different sexes. A 10-year-old boy with a BMI of 23 is classified as obese, whereas a 15-year-old boy with the same BMI is healthy. Therefore, simply calculating the BMI does not yield a reliable result. The BMI must be compared against age and sex growth charts. The reason for these differences lies in the fact that children develop more muscle as they age and that boys tend to have more muscle than girls. Normal BMI for boys ranges from 13.8-16.8 for a 5-year-old, 14.2 to 19.4 for a 10-year-old, and 16.5-23.4 for a 15-year-old. Normal girl BMIs include 13.6 to 16.7 for a 5-year-old, 14 to 19.5 for a 10-year-old, and 16.3 to 24 for a 15-year-old.

Starting at 25.0, the higher your BMI, the greater is your risk of developing obesity-related health problems. These ranges of BMI are used to describe levels of risk:

  • Overweight (not obese), if BMI is 25.0 to 29.9
  • Class 1 (low-risk) obesity, if BMI is 30.0 to 34.9
  • Class 2 (moderate-risk) obesity, if BMI is 35.0 to 39.9
  • Class 3 (high-risk) obesity, if BMI is equal to or greater than 40.0


Pay Attention to Low Numbers

Don't ignore a low BMI number. Low BMI percentiles can indicate health problems as well, including malnutrition, which can lead to a weakened immune system, decreased rate of growth, increased risk of injury and increased risk of respiratory problems.