What Is BMI? BMI (body mass index) is a calculation that estimates how much body fat a person has based on their weight and height. The doctor checks BMI at all routine checkups. Because BMI changes with age, doctors plot BMI measurements on standard gender-specific growth charts. Over several visits, the doctor is able to track your growth pattern. BMI is not a direct measure of body fat. Teens can have a high BMI if they have a large frame or a lot of muscle, not excess fat. And a person with a small frame may have a normal BMI but still can have too much body fat. Also, it's common for teens to gain weight quickly — and see their BMI go up — during puberty. Your doctor can help you figure out whether this weight gain is a normal part of development or whether it's something to be concerned about. The categories that describe a person's weight are: Underweight: BMI is below the 5th percentile age, gender, and height. Healthy weight: BMI is equal to or greater than the 5th percentile and less than the 85th percentile for age, gender, and height. Overweight: BMI is at or above the 85th percentile but less than the 95th percentile for age, gender, and height. Obese: BMI is at or above the 95th percentile for age, gender, and height. It's important to look at BMI as a trend instead of focusing on individual numbers. Any one measurement, taken out of context, can give you the wrong impression of your growth. Our BMI calculator can help you find out what your BMI is. When you get your results, it's a good idea to ask a doctor or health professional to help you interpret the results. Although not a perfect measure of body fat, BMI can help identify teens who are gaining weight too slowly or too quickly. If you are concerned about your weight, talk to your doctor. Back to Articles Related Articles Nutrition & Fitness Center Visit our nutrition and fitness center for teens to get information and advice on food, exercise, and sports. Read More The Deal With Diets With all the focus on dieting, how do you figure out what's healthy and what isn't? Read this article to get the basics on dieting. Read More 5 Ways to Reach a Healthy Weight Most dieters regain the weight they lost by dieting when they go back to their old eating habits. Get our tips on the best ways to drop excess weight. Read More What's the Right Weight for My Height? One of the biggest questions guys and girls have is whether they're the right weight. Because the body is growing and changing so much during adolescence, it can be tough to answer this question. Read More Staying at a Healthy Weight Here are some practical, everyday tips on making exercise and healthy eating work for you instead of feeling like it's the other way around. Read More How Can I Lose Weight Safely? Lots of people are unhappy with their present weight, but aren't sure how to change it - or even if they need to. Get the facts on weight loss here. Read More Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.