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Child Body Mass Index

The formula for calculating BMI is same for kids as well as adults, but the difference lies in the interpretation of the results. Let us take a look at the method of BMI calculation in kids.
Sonia Nair
Body mass index or BMI has become a household term for adults. With the rise in the number of overweight children, more and more people are taking interest in weight management of their kids. Usage of BMI for kids has improved significantly in recent years.
Even though, the body mass index for kids is calculated using the same formula as in adults; unlike the latter, the BMI charts for children are based on their age and gender. In adults, the same chart applies for all, whereas in kids, it differs for boys and girls, and for various age groups.

How to Calculate BMI for Kids

Weight classification in kids depends on their age and gender. Child body mass index is calculated by dividing the body weight in pounds with the square of the height in inches, and the resultant value is multiplied with 703. It is very important to obtain the accurate height and weight of a child in order to calculate the BMI.
The value obtained is the BMI, which is interpreted using a BMI-for-age growth chart. The chart provides the percentile ranking, which denotes the relative position of the BMI of your child, in comparison to other kids of same age and gender.
The BMI of any child can be evaluated on the basis of 'BMI-for-age percentile'. Percentile is the value below which a certain percent of observations fall. For example, if a child belongs to the 75th percentile, as per his/her age, gender, and BMI; it means that 75% of kids of the same age and gender have a lower BMI.
The percentile is interpreted on the basis of the age and gender of the kids. This criterion for classification is based on the fact, that the amount of body fat changes with age and gender. Hence, child body mass index is often referred to as BMI-for-age.
Many organizations (like, Centers for Disease Control and Prevention) have developed specialized charts for this purpose. The percentile for a particular BMI is calculated as per the chart, and the weight status is determined according to the category of percentile.
The categories are as follows:
  • If the corresponding value of the BMI in the chart is below the 5th percentile, your child is considered underweight.
  • If it comes between 5th to 85th percentile, the weight of your child is normal for his/her age.
  • A value that lies between 85th to 95th percentile shows a risk of overweight.
  • A percentile which is equal to or above 95 indicates overweight.
This criterion is applicable for kids (as well as teens) in the age group 2 to 19; and is helpful in determining their weight status.
The third category, which indicates the 'risk of overweight' is found to be confusing for many people. As per some charts, the third category is classified as overweight and the fourth as obese; but many others omit the word 'obese', not to hurt the feelings of kids. Hence, the third category should be understood as 'being on the verge of overweight' or 'overweight' itself, and the fourth category, which is classified as 'overweight' is actually equivalent to 'obesity'.
Kids who look fat, may not be overweight according to the BMI-for-age percentile, and vice versa. This chart is considered a reliable tool to determine the weight status of kids as well as teens.