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Body Mass Index (BMI):

A Guide for Teens



  • BMI provides an estimate of a person's body fat.
  • BMI charts are used to see if you are underweight, normal, or overweight compared to other teens.
  • A high BMI can mean that a person is at risk for health-related problems such as high blood pressure or diabetes.




















You may have learned about BMI in health class, heard your health care provider talk about your BMI, or you may have gotten a BMI report card from your school. BMI, short for body mass index, is getting a lot of attention lately because some schools in the United States are measuring students BMI and sending home BMI report cards. This guide was created to help you understand what BMI is, why and how it is used, and when to talk with your health care provider about your BMI.


What is Body Mass Index (BMI)?

BMI is used to estimate a person’s body fat. Everyone needs some body fat, but having too much body fat can put a person at risk for health problems such as high blood pressure and diabetes. Having too little body fat is also unhealthy.


How is BMI measured?

BMI is a mathematical calculation of your weight (in pounds) times 703 divided by your height (in inches) squared. You can figure out your BMI on your own using the formula below:

BMI Calculation Formula


You can also figure out your BMI using this link; but, it is best to ask your health care provider or school nurse for this information to make sure your height and weight are measured correctly.


What does my BMI mean?

Once your health care provider or school nurse calculates your BMI, they must use a special chart to convert this number into a percentile. Your percentile tells you how your BMI compares with other teens your age and sex. Your health care provider can tell you if your BMI percentile falls into the underweight, normal weight, overweight or obese category.


BMI percentile Category
less than 3rd percentile Underweight
between 5th and 85th percentile Normal Weight
between 85th and 95th percentile Overweight
greater than 95th percentile Obese


No matter what your BMI percentile is, it's important to ask if it has changed over the last couple of years. Although BMI can change when you're growing or going through puberty, teens should ideally stay at about the same BMI percentile during their teen years. If your BMI percentile changes a lot, you should talk to your health care provider about possible reasons for the change.


What is a BMI report card and what is it used for?

Some schools in the United States are checking weights and heights of students at school, calculating BMI, and sending this information home to parents as a BMI report card. The report cards are used to identify teens who are underweight, overweight, or obese. Teens who fall in any of these groups may be at a higher risk for weight-related health problems.


What if my BMI is high?

Talk to your health care provider about whether you should be concerned about your BMI and about healthy ways to manage your weight. Keep in mind that BMI is just an estimate of body fat. Most teens who have a BMI in the overweight or obese category also have a high amount of body fat, but there are some who don't. This is true for muscular teens (especially those who work out or are very athletic and toned). Their weight may be higher because of extra "muscle" and not because of extra body fat.


If your BMI is falls in the underweight, overweight, or obese category, you should see your health care provider. He/she can check to see if you are at risk for getting any serious health problems and help you work on a healthy weight management plan, if needed.



Nihiser AJ, et al. Body mass index measurement in schools. Journal of School Health.
2007; 77(10): 651-671.


Written and reviewed by the CYWH Staff at Boston Children's Hospital


Updated: 8/9/2012


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