# Body Mass Index for Child or Teen Calculator

Since 1980, the percentage of children who are obese has more than tripled in the U.S., from 5.5% to 17% and a similar trend is occurring worldwide. Overweight children are getting so prevalent that they are now perceived by some as normal. The extra weight can pose significant health risks during the development years and into adulthood. There is an 80 % chance that they may stay overweight for their entire lives.

For adults, Body Mass Index (BMI) is commonly used as a preliminary diagnoses in identifying health risks associated with excessive weight. It is a simple but useful numeric measurement of fatness or thinness.

The index is based on a person's height and weight using the following formula:

BMI = kg / m^{2}

where *kg* is a person's weight in kilograms and *m ^{2}* is their height in metres squared.

Excessive weight is evaluated differently for children and teens. Unlike BMI for adults, age and sex is taken into consideration because boys and girls grow at different rates and differ in their amount of body fat as they grow. Consequently, so does their relation to body fatness. For children between 2 and 20 years of age, BMI is calculated using the same formula as adults but their BMI is compared with growth charts. From the growth chart, a BMI percentile is produced from the child's BMI, age and sex. Their BMI is often referred to as BMI-for-age.

BMI-for-age percentile is a useful indicator of how your child’s weight compares with other children of the same age and gender. If for example, your child is in the 80th percentile, it means that your child’s weight is greater than that of 80% of the other children of the same age and gender.

This calculator first calculates your child's BMI then looks up the corresponding BMI-for-age percentile for your child's sex and age. The percentile is derived from the *Centers for Disease Control and Prevention* growth charts data.

The table shown below are the BMI-for-age percentile ranges and corresponding weight categories. A BMI-for-age percentile between the 5th and 85th is considered a healthy weight.

Percentiles | Category |
---|---|

less than 5th | Underweight |

5th to 85th | Healthy Weight |

85th to 95th | Overweight |

95th or greater | Obese |

Note that neither BMI nor growth charts measure body fat. Having a BMI between the 85th and 95th percentile could mean that the excessive weight is due to muscle and may not be all attributed to fat. Being athletic or having lean builds will show a high BMI without necessarily having excessive fat. Studies have found that as much as 15% of children fall into this category. However, in almost all children with percentiles above the 95th, the higher weight was found to be due to mostly fat.