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.