What Is BMI? Body mass index (BMI) is a calculation that uses height and weight to estimate how much body fat someone has. You can use the KidsHealth BMI calculator below to find your child's BMI. But it's also important to talk to your child's doctor to help understand the results. Tracking BMI Starting when your child is 2 years old, the doctor will determine BMI at all routine checkups. Because BMI changes with age, doctors plot children's BMI measurements on standard gender-specific growth charts. Over several visits, the doctor is able to track your child's growth pattern. Although not a perfect measure of body fat, BMI helps identify children who are gaining weight too slowly or too quickly. What Do the Figures Mean? BMI percentiles show how a child's measurements compare with others the same gender and age. For example, if a child has a BMI in the 60th percentile, 60% of the kids of the same gender and age who were measured had a lower BMI. BMI is not a direct measure of body fat. Kids can have a high BMI if they have a large frame or a lot of muscle, not excess fat. And a kid with a small frame may have a normal BMI but still can have too much body fat. BMI is less accurate during puberty. It's common for kids 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 the 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 child's growth. While BMI is an important indicator of healthy growth and development, BMI is not a perfect measure of body fat. If you're concerned that your child may be gaining or losing weight too fast, talk to your doctor. Back to Articles Related Articles Your Child's Weight "What's the right weight for my child?" is one of the most common questions parents have. It seems like a simple one, but it's not always easy to answer. Read More Growth Charts Doctors use growth charts to figure out whether kids' height and weight measurements are "normal" and whether they're developing on track. Here are some facts about growth charts. Read More Overweight and Obesity Preventing kids from becoming overweight means making choices in the way your family eats and exercises, and how you spend time together. 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