The Basics

Written by the doctors and editors at UpToDate

What is body mass index?

Body mass index, or "BMI," is a calculation doctors use to help understand a person's health. Based on this number, a person falls into 1 of these categories:

● Underweight

● Healthy weight

● Overweight

● Having obesity

For children, the definition of a healthy BMI depends on their age and sex.

How do I know if my child has a healthy BMI?

When your child sees their doctor or nurse for regular check-ups, this will include checking their height and weight. The doctor or nurse uses these measurements to calculate their BMI.

Your child's BMI is then compared with a "growth chart." These charts have information about the typical heights, weights, and BMIs of children of the same age and sex. Doctors use these charts to see if a child is underweight, a healthy weight, overweight, or has obesity.

Does my child's weight affect their health?

Yes. It's important to have a healthy weight, because children who are very overweight can have:

● Asthma – This is a lung condition that can make it hard to breathe.

● High blood pressure

● Sleep apnea – This is a condition that makes people stop breathing for short periods during sleep.

● Knee or back pain

● Liver problems

● Diabetes

Having a healthy weight as a child also makes a person more likely to have a healthy weight later in life. Being overweight as a teen or adult can lead to medical problems such as high blood pressure, diabetes, heart attacks, and some types of cancer.

What causes children to gain too much weight?

Children can gain too much weight for different reasons. Some children simply gain weight more easily than others. This means they have to work harder to have a healthy weight. This can run in families. Many other things can cause a person to gain extra weight, including where they live, their medicines, certain health conditions, or stress.

For some children, things that might increase the risk of weight gain include:

● Eating snack foods or fast foods too often

● Drinking a lot of sugary drinks, like soda and juice

● Not getting enough physical activity

● Spending a lot of time on a computer or smartphone, watching TV, or playing video games

● Not getting enough sleep

There are many reasons why a child might have these habits. For example, they might eat less healthy foods because they are used to them, or because they do not have access to healthier foods. Depending on things like work, school, and where they live, it can be hard for a child to exercise or get enough sleep.

Will my child need tests?

Maybe. The doctor or nurse will talk with you and your child, and do an exam. They might do blood tests to check for:

● Health problems that can happen when children are overweight

● A condition that could be causing your child to gain weight easily

How can I help my child get to a healthier weight?

Help them eat healthy foods and be more active. Making these lifestyle changes can be hard, especially at first. It helps to keep a regular routine for eating and physical activities.

Some goals you can work toward to help your child be healthier:

● Offer your child 5 or more servings of fruits or vegetables each day. Fresh or cooked fruits and vegetables count toward the goal, but fruit juice does not. A serving is usually 1 whole fruit (such as an apple or banana) or a half cup of vegetables. If you don't have fresh fruits and vegetables available, you can use frozen or canned ones instead.

If your child does not like vegetables or fruit, start slowly. Eat these foods yourself to set a good example, and have your child keep trying them.

● Help your child be physically active for 1 hour or more each day. This can include organized activities like sports or dance. But children can also get exercise just through play.

● Have a family "media plan" for each person in your home, including media content and time limits. "Media" includes using social media, watching TV, playing video games, and using screens for things other than schoolwork. Talking about media use and knowing what your child is doing can help keep them safe. Setting limits also gives your child more time for other activities, like physical activity and spending time with friends.

Experts recommend young children (ages 2 to 5) have no more than 1 hour a day of screen time. Older children should also limit screen time.

● Do not give your child any sugary drinks. These include soda, sports drinks, and all juices.

You and your child might not be able to meet all these goals at first, but that's OK. Choose 1 or 2 goals to try first. Later, you can try to meet all these goals.

How else can I help my child be healthy?

You should:

● Try to avoid bringing a lot of unhealthy food into your home. If you have unhealthy food in the home, your child is likely to eat it even if you tell them not to. Unhealthy foods include chips, cookies, pastries, and many other snack foods. It's OK for your child to eat these foods once in a while, but not too often.

● Make sure your child gets enough sleep. Some studies suggest children who do not sleep enough are more likely to gain too much weight. In general, children 3 to 5 years old should get 10 to 13 hours of sleep (including naps). Older children should get 9 to 12 hours of sleep each night, and teens should get 8 to 10 hours.

To encourage sleep, have a set bedtime and wake-up time every day, even for non-school days. It's also a good idea to have a quiet before-bed routine. Try not to let your child watch TV or play video games right before bedtime.

● Involve the whole family. Have everyone in your home eat healthier and be more active, even those who have a healthy weight. Try to do physical activities together. This can be as simple as going to a park or playground or taking a walk.

● Tell your child the goal is to be healthy and strong. Tell them an important way to be healthy and strong is to eat healthy food and be active. Try not to focus too much on their weight or how they look.

● Get help if your child's weight is causing them to be sad or worried or have a hard time in school. Ask the doctor or nurse for ways to get help for your child.

● Work with your child's doctor or nurse. Have regular check-ups, so the doctor or nurse can follow your child's BMI and health over time. Tell them if you are having trouble meeting the above goals. They can help you get started or give you some tips. They might also recommend you talk with a food expert called a "dietitian." They can help you choose healthy foods and plan meals.

All topics are updated as new evidence becomes available and our peer review process is complete.

This topic retrieved from UpToDate on: Mar 22, 2026.

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