"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.

What Is Body Mass Index?

Body mass index (BMI) is a formula that doctors use to estimate how much body fat someone has. The BMI formula uses height and weight measurements to calculate a BMI number. Though the formula is the same for adults and children, figuring out what the BMI number means is a little more complicated for kids.

For kids, BMI is plotted on a growth chart because what is normal changes with age. Different BMI charts are used for boys and girls because growth rate and the amount of body fat differs between boys and girls. Each BMI chart is divided into percentiles that compare measurements with children the same age and gender.

The categories that describe a person's weight are:

Underweight: BMI 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 at or above the 85th percentile but less than the 95th percentile for age, gender, and height.

Obese: BMI at or above the 95th percentile for age, gender, and height.

Before you calculate 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.

Talking to the Doctor About BMI Results

You can calculate BMI on your own, but talk to your doctor about what it means. Doctors use more than a BMI to assess a child's current weight. They also consider the stage of puberty and the child's past BMI results to track growth. Spotting trends early can be helpful before a child's weight loss or weight gain become a problem.

Overweight kids and teens can develop weight-related health problems, such as type 2 diabetes, high cholesterol, and high blood pressure and are more likely to be overweight as adults. And adults who are overweight have a greater chance of serious health problems, 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 (like a bodybuilder or athlete) can have a high BMI but not too much fat. 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 child's BMI with your doctor.

When Kids Are Overweight or Underweight

If you think your child has gained too much weight or is too thin, talk to their doctor. The doctor has measured your child's height and weight over time and knows whether growth is going as it should.

If the doctor is concerned about your child's height, weight, or BMI, they may ask questions about your child's health, physical activity, and eating habits, and your family's medical history. The doctor can put all this information together to tell if there's a weight or growth problem.

If your child has a high BMI, the doctor may give you diet and exercise recommendations. Focus on healthy behaviors, like eating more fruit and vegetables and being active every day. Doctors also can prescribe medicine or surgery for some kids.

What if your child is underweight? Most kids who weigh less than others their age are just fine. They may be going through puberty later than their peers. 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. But talk with the doctor if your child:

• is losing weight
• is tired or ill a lot
• has a lasting cough, fever, diarrhea, or other symptoms
• has signs of an eating disorder