"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.
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. Second, people have different body types. Some are more muscular or shaped differently than others.
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 BMI.
People Grow and Develop Differently
Not everyone grows and develops on the same schedule, but most people go through a period of faster growth during their teens. During puberty, the body begins making hormones that spark physical changes like faster muscle growth (particularly in guys) and spurts in height. As the amount of muscle, fat, and bone in the body changes during this time, some people might gain weight more rapidly.
It can feel strange adjusting to a new body. But all that new weight gain can be perfectly fine — as long as body fat, muscle, and bone are in the right proportion.
Because weight 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 the body mass index, or BMI. BMI is a formula that doctors use to 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 helps tell a person whether he or she is in the underweight, healthy weight, overweight, or obese range.
The growth charts have lines for "percentiles." Like percentages, percentiles go from 0 to 100. The eight lines on the BMI growth charts show the 5th, 10th, 25th, 50th, 75th, 85th, 90th, and 95th percentiles. The 50th percentile line is the average BMI of the teens who were measured to make the chart.
When your BMI is plotted on the chart, the doctor can see how you compare with other people the same age and gender as you. Based on where your number is on the chart, a doctor will decide if your BMI is in the underweight, normal weight, , or range.
There's a big range of normal on the chart: Anyone who falls between the 5th percentile and the 85th percentile is a healthy weight. If someone is at or above the 85th percentile line on the chart (but less than the 95th percentile), doctors consider that person overweight. A BMI measurement over the 95th percentile line on the chart puts someone in the obese range.
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.
A doctor can use BMI results from past years to track whether you may be at risk for becoming overweight. Spotting this risk early on can be helpful because the person can then make changes in diet and exercise to help head off a weight problem.
BMI can be a good indicator of a person's body fat, but it doesn't always tell the full story. People can have a high BMI because they have a large frame or a lot of muscle (like a bodybuilder or athlete) instead of excess fat. Likewise, a small person with a small frame might have a normal BMI but could still have too much body fat. These are other good reasons to talk about your BMI with your doctor.
How Can I Be Sure I'm Not Overweight or Underweight?
If you think you've gained too much weight or you're too skinny, a doctor can help you decide whether it's normal for you or whether you really have a weight problem. Your doctor has measured your height and weight and has plotted your BMI over time. So he or she can tell whether you're growing normally.
If your doctor is concerned about your height, weight, or BMI, he or she may ask questions about your health, physical activity, and eating habits. Your doctor also may ask about your family background to find out if you've inherited traits that might make you taller, shorter, or a late bloomer (someone who develops later than other people the same age). 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 just fine. You might be going through puberty on a different schedule than some of your peers, and your body may be growing and changing at a different rate. Most underweight teens catch up eventually and there's rarely a need to try to gain weight.
In a few cases, teens can 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, stomachache, diarrhea, or other problems that have lasted for more than a week or two.
Some people are underweight because of eating disorders, like anorexia or bulimia, that they need to get help for.
Getting Into Your Genes
Heredity plays a role in body shape and what a person weighs. People from different races, ethnic groups, and nationalities tend to have different body fat distribution (meaning they have fat in different parts of their bodies) or body composition (their amounts of bone and muscle versus fat).
But genes are not destiny. No matter whose genes you inherit, you can have a healthy body and keep your weight at a level that's normal for you by eating right and being active.
Genes aren't the only things that family members may share. Unhealthy eating habits can be passed down, too. The eating and exercise habits of people in the same household may have an even greater effect than genes on a person's risk of becoming overweight.
If your family eats a lot of high-fat foods or snacks or doesn't get much exercise, you may tend to do the same. 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.
It can be tough dealing with the physical changes your body goes through during puberty. But at this time, more than any other, it's not a specific number on the scale that's important. It's keeping your body healthy — inside and out.