As a teenager, you’ve probably heard the term “BMI” plenty of times. Maybe your school even gave you a BMI report card. But what exactly is BMI? And is it something that you, as a teen, even need to think about?

BMI stands for “Body Mass Index,” a measurement doctors use to estimate whether people are underweight, normal weight, overweight, or obese. Is BMI important for teens? Yes—because if your BMI is too high or too low, you could be at risk for disease.

Where the BMI number comes from

Doctors use a mathematical formula to calculate body mass index for teens. The formula is a little complicated. It’s a person’s weight (in kilograms) divided by height (in meters, squared), multiplied by 703. The calculation for teens (and kids 2-19) also takes age into account, and whether are male or female. If you’re into math, you can certainly figure it out yourself, but there are plenty of online tools that will figure it out for you. The BodiMojo web site has its own BMI calculator. You can also download a free BodiMojo BMI calculator app, called mojo BMI, right to an iPod touch or iPhone.

What the number means

When using BMI to evaluate weight in adults, health experts recommend a BMI of 25 or less. For children and teens, the BMI is still calculated using height and weight, but then needs to be plotted on a “body mass index for age” male or female growth chart with growth curves on it ranging from the 5th percentile to the 95th percentile.

Where on the curve your BMI will fall is dictated by your age, so, for example, the same BMI in a 13 year old or 15 year old teenager will fall on two different places on the curve.

Once your BMI is plotted on the curve you can then determine if it is high, low or just right.  According to the Centers for Disease Control and Prevention, the following numbers suggest whether you may be underweight, at a healthy weight, overweight or obese. Someone in the overweight or obese category may be at higher risk for health problems due to their weight:

Just a Number

BMI alone can’t describe how healthy you are overall. If you’re eating nutritious foods, getting regular exercise, and your doctor thinks you’re doing OK, that’s what really matters!

To see what these weight charts look like, and maybe plot your own BMI, go to the growth chart for boys or for girls.

While plotting your BMI is an important part of your health assessment, it can only give you a rough idea of whether you’re overweight or underweight. There’s more to it than that. The key question is how much body fat you have, which just looking at your BMI can’t tell you.

For instance, a very athletic teen may have what seems like a high BMI, but that could simply mean his weight is relatively high because of all of his muscle—which weighs more than fat. In this case, a high BMI is not necessarily a bad thing. If you have concerns, discuss your BMI percentile rank with your doctor, who will take into account several different factors when evaluating your weight.

What to do if your BMI is high or low

If your BMI (and your doctor’s opinion) indicates you’re overweight or obese for a teenager, you could be at risk for a number of health problems, including Type 2 diabetes, heart disease, gallstones, or stroke. To avoid such issues, rethink your eating habits and get more exercise.

If you’re underweight? Find healthy ways to gain weight and consider building up your muscles.

Interestingly, teens at risk for eating disorder often fall into a normal BMI range and there are different signs and symptoms for teens at risk for problem eating or weight management issues.

Whatever your BMI, there are plenty of great ideas on how to eat right and exercise to keep your body healthy at BodiMojo.com.

Hillary Wright, RD, contributed to this article.