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What Is BMI?

Body mass index (BMI) is a calculation that uses height and weight to estimate how much body fat someone has. You can use the KidsHealth BMI calculator below to find your child's BMI. Also talk with your child's doctor to help understand the results.

Tracking BMI

Starting at age 2, doctors determine BMI at all routine checkups. Because BMI changes with age, doctors plot it on growth charts. Over time this shows your child's growth pattern. Different BMI charts are used for each gender because growth rate and the amount of body fat differ between genders.

Many schools have added BMI to students’ annual health screenings. Parents may get a BMI report card, letting them know if there are any concerns. Share the BMI information with your child's doctor, who can look at the results and offer advice.

BMI isn’t perfect, but it helps spot kids who are gaining weight too slowly or too quickly.

What Do the Figures Mean?

BMI percentiles show how a child's measurements compare with others of the same age and gender. For example, if a child has a BMI in the 60th percentile, 60% of the kids of the same age and gender who were measured have a lower BMI.

Categories that describe a person's weight include:

  • Underweight: BMI is below the 5th percentile.
  • Healthy weight: BMI is equal to or greater than the 5th percentile and less than the 85th percentile.
  • Overweight: BMI is at or above the 85th percentile but less than the 95th percentile.
  • Obese: BMI is at or above the 95th percentile.

Things to Keep in Mind

  • BMI doesn’t tell the full story. Kids can have a high BMI if they have a large frame or a lot of muscle (like athletes), not excess fat. And a kid with a small frame may have a normal BMI but still can have too much body fat.
  • It’s less accurate during puberty. It's common for kids to gain weight quickly — and see their BMI go up — during puberty. Your doctor can help you figure out whether this weight gain is normal.
  • BMI should be looked at as a trend. Any one measurement, taken out of context, can give you the wrong impression of your child's growth. Spotting trends early can be helpful before a child's weight loss or weight gain becomes a problem.

If you're concerned that your child has gained too much weight or is too thin, talk with your doctor.

When Kids Are Overweight or Underweight

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

High BMI

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, like heart disease.

If your child has a high BMI, the doctor may give diet and exercise recommendations. Doctors also can prescribe medicine or weight-loss surgery for some kids.

Low BMI

Most kids who are underweight are fine. They may go through puberty later than their peers. Most teens who are underweight 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:

Tips to Help Kids Maintain a Healthy Weight

  • Encourage kids to get 60 minutes or more of physical activity each day.
  • Offer fruits and vegetables at meals and snacks and encourage your child to eat five or more servings a day.
  • Serve the right portion sizes for your child's age.
  • Limit sugary drinks and offer low-fat milk or water instead.
  • Keep screen time (time spent watching TV, playing video games, using a smartphone, computer, or other device) to less than two hours a day.

Set a good example and have your whole family follow these tips as well.

Date reviewed: August 2026