"What's the right weight for my height?" is one of the most common questions girls and guys have. It seems like a simple question. But for teens, it's not always an easy one to answer. Not everyone grows and develops on the same schedule.

It's normal for two people who are the same height and age to have very different weights. First, not everyone goes through puberty at the same time. Some kids start developing as early as age 8 and others might not develop until age 14. During puberty, the body begins making hormones that spark physical changes like faster muscle growth (particularly in guys), spurts in height, and weight gain. Second, people have different body types. For example, some are muscular and large framed while others are thinner with smaller frames.

For these reasons, you can't point to a number on a scale as the "right" number. But it is possible to find out if you are in a healthy weight range for your height and age. That's why doctors use body mass index, or BMI.

Figuring Out BMI

Because weight gain is more complicated during our teens, doctors don't rely on weight alone to figure out if someone is in a healthy weight range. Instead, they use BMI. BMI helps doctors estimate how much body fat a person has based on his or her weight and height.

The BMI formula uses height and weight measurements to calculate a BMI number. This number is then plotted on a BMI chart, which has lines called percentiles. BMI percentiles show how a teen's measurements compare with others the same gender and age. For example, if a teen has a BMI in the 60th percentile, 60% of teens the same gender and age had a lower BMI.

The categories that describe a person's weight are:

• Underweight: BMI is below the 5th percentile for 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 numbers 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.

What Does BMI Tell Us?

You can calculate BMI on your own, but it's a good idea to ask your doctor, school nurse, or other health professional to help you figure out what it means.

BMI is not a direct measure of body fat, and it doesn't always tell the full story. People can have a high BMI because they have a lot of muscle (like a bodybuilder or athlete) instead of excess fat. Likewise, a person with a small frame might have a normal BMI but could still have too much body fat.

How Can I Be Sure I'm Not Overweight or Underweight?

If you think you've gained too much weight or are too skinny, a doctor can help you know if it's normal for you or whether you do have a weight problem. At each visit, your doctor measures your height and weight and plots your BMI. He or she uses those measurements over time to tell whether you're growing as expected.

If your doctor is concerned about your height, weight, or BMI, they may ask questions about your health, physical activity, and eating habits. Your doctor also may ask about your family background to find out if being tall, short, or a late bloomer (someone who develops later than other people the same age) runs in your family. The doctor can then put all this information together to decide whether you might have a weight or growth problem.

If your doctor thinks you're overweight, he or she may refer you to a dietitian or doctor specializing in weight management. These experts can offer eating and exercise recommendations based on your individual needs. Following a doctor's or dietitian's plan that's designed especially for you will work way better than following fad diets.

What if you're worried about being too skinny? Most teens who weigh less than other people their age are healthy. People in your family may be small or thin, or you might be going through puberty later than some of your peers, or your body may be growing at a slower rate. Most underweight teens catch up and there's rarely a need to try to gain weight.

Sometimes, teens may be underweight because of a health problem that needs treatment. See a doctor if you notice any of these things:

• You feel tired or ill a lot.
• You have a cough, diarrhea, poor appetite, or other problems that have lasted for 2 weeks or more.
• You are losing weight.

Some people may be underweight because of an eating disorders, like anorexia or bulimia. Talk to your doctor if you think you may have an eating disorder.

Getting Into Your Genes

Heredity plays a role in body shape and what a person weighs. Body shape and weight tends to run in families. So family members may have similar:

• body types: they have fat in certain parts of their bodies
• body composition: their amounts of bone and muscle versus fat

Genes aren't the only things that family members may share. Eating and physical activity habits can be passed down too. If your family eats a lot of high-fat foods or snacks or doesn't get much exercise, you may do the same.

But genes are not destiny. The good news is these habits can be changed for the better. Even simple changes like walking more or taking the stairs can benefit a person's health. No matter what genes you inherit, you can be healthy and be the weight that's right for you eating a balanced diet and being active every day.