"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. People have different body types, so no single number is the right weight for everyone.

Among kids the same height and age, some are more muscular or more developed than others. That's because not all kids have the same body type or develop at the same time.

It is possible to find out if kids are in a healthy weight range for their height, though, with a little effort.

Growth and Puberty

Not everyone grows and develops on the same schedule, but teens do go through a period of faster growth. During puberty, the body begins making hormones that spark physical changes like faster muscle growth (particularly in boys) and spurts in height and weight gain in both boys and girls. Once these changes start, they continue for several years. The average person can expect to grow as much as 10 inches (25 centimeters) during puberty before reaching full adult height.

Most kids gain weight more rapidly during this time as the amounts of muscle, fat, and bone in their bodies changes. All that new weight gain can be perfectly fine — as long as body fat, muscle, and bone are in the right proportion.

Because some kids start developing as early as age 8 and some not until age 14 or so, it can be normal for two kids who are the same height and age to have very different weights.

It can feel quite strange for kids to adjust to suddenly feeling heavier or taller. So it's perfectly normal for a child to feel self-conscious about weight during adolescence — many kids do.

Figuring Out Fat Using BMI

Experts have developed a way to help figure out if someone is in the healthy weight range for his or her height. It's called 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 chart, which tells a person whether he or she is underweight, average weight, at risk of becoming overweight, or overweight.

Figuring out the body mass index is a little more complicated for teens than it is for adults (that puberty thing again). BMI charts for kids and teens use percentile lines to help individuals compare their BMIs to those of a very large group of people the same age and gender. There are different BMI charts for boys and girls under age 20.

BMI numbers are plotted on the chart according to a person's age and gender. Each BMI chart has eight percentile lines for 5th, 10th, 25th, 50th, 75th, 85th, 90th, and 95th percentiles. A child whose BMI is at the 50th percentile is close to the average of the age group. A child above the 95th percentile is considered overweight because 95% of the age group has a BMI less than he or she does. A child below the 5th percentile is considered underweight because 95% of the age group has a higher BMI.

Before you measure your child's BMI, you'll need an accurate height and weight measurement. Bathroom scales and tape measures aren't always precise. So the best way to get accurate measurements is by having kids weighed and measured at a doctor's office or at school.

What Does BMI Tell Us?

You can calculate BMI on your own, but consider asking your doctor to help you figure out what it means. Doctors do more than just use BMI to assess a child's current weight. They also take into account where a child is during puberty and use BMI results from past years to track whether that child may be at risk for becoming overweight. Spotting this risk early on can be helpful because kids can then make changes in diet and exercise before they go on to develop a weight problem.

No one likes looking overweight, but weight problems can be more serious than just appearance. People who are overweight as teens increase their risk of developing health problems, such as diabetes and high blood pressure. They're also more likely to be overweight as adults. And adults who are overweight may develop other serious health conditions, such as heart disease.

Although BMI can be a good indicator of body fat, it doesn't always tell the full story. Someone with a large frame or a lot of muscle instead of excess fat (like a bodybuilder or athlete) can have a high BMI. Likewise, a small person with a small frame may 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.

When Kids Are Overweight or Underweight

If you think your child has gained too much weight or is too skinny, a doctor should help you decide whether there's a true weight problem. Your doctor has measured your child's height and weight over time and knows whether growth is proceeding normally.

If concerned about your child's height, weight, or BMI, the doctor may ask questions about your child's health, level of physical activity, and eating habits, as well as your family background (to find out if your child has inherited traits that might make him or her taller, shorter, or a late bloomer). The doctor can put all this information together to determine whether there's a weight or growth problem.

If your doctor thinks your child's weight isn't in a healthy range, you will probably get specific dietary and exercise recommendations. Following a doctor's or dietitian's plan that's designed for your child will work much better than following fad diets. For kids and teens, fad diets or starvation plans can actually slow down growth and sexual development, and the weight loss usually doesn't last.

What if your child is worried about being too skinny? Most teens who weigh less than other teens their age are just fine. They may be going through puberty on a different schedule than some of their peers, and their bodies might be growing and changing at a different rate. Most underweight teens catch up in weight as they finish puberty during their later teen years, and there's rarely a need to try to gain weight.

In a few cases, kids and teens can be underweight because of a health problem that needs treatment. If your child feels tired or ill a lot, or has symptoms like a cough, stomachache, diarrhea, or other problems that have lasted for more than a week or two, talk with your doctor. Some kids and teens are underweight because of eating disorders, like anorexia or bulimia, that require attention.

The Role of Genes

Heredity plays a role in a person's body shape and weight. People from different races, ethnic groups, and nationalities tend to have different body fat distribution (meaning they accumulate fat in different parts of their bodies) or body composition (amounts of bone and muscle versus fat). But genes are not destiny — kids can reach and keep a healthy weight by eating right and being active.

Genes aren't the only things that family members may share. It's also true that unhealthy eating habits can be passed down. The eating and exercise habits of people in the same household probably have an even greater effect than genes on someone's risk of becoming overweight. If your family eats a lot of high-fat foods or snacks or doesn't get much exercise, your kids may tend to do the same. The good news is these habits can be changed for the better. Even simple forms of exercise, such as walking, have huge health benefits.

So remember, it's not a specific number on the scale that's important. It's making sure that kids stay healthy — inside and out.

Reviewed by: Steven Dowshen, MD
Date reviewed: May 2005