What is Underweight for a 13-Year Old?

Doctors are consistently monitoring weight, especially during childhood and teenage years, because it is a helpful tool in determining if someone is developing properly. Everyone develops and grows a little bit differently so it is important to look at trends in weight over time, rather than just a one-time measurement. 

A 13-year old boy who is 61 inches tall (5 foot 1 inch or 155 cm) would be considered underweight below 82 pounds (37 kg). A 13-year old girl who is 61 inches tall (5 foot 1 inch or 155 cm) would be considered underweight below 81 pounds (37 kg). Other ages and heights would be classified differently.

The Center for Disease Control and Prevention (CDC) has decided that a teenager is “underweight” if their body mass index (BMI) is below the 5th percentile when plotted on growth charts. Since this measurement only accounts for height and weight, it is important to also look at genetic factors, medical history, eating habits, physical activity level, stress levels, and other relevant factors and behaviors.

Read on for more information about what is a healthy weight for a 13-year old, if it is unhealthy to be underweight, who should gain weight, tips for healthy weight gain, and how parents can be supportive and encouraging to their children while helping them maintain a healthy weight.

What is a Healthy Weight for a 13-Year Old?

The range of what is considered a “healthy weight” is pretty large. The CDC uses the following classifications to determine if someone is underweight, healthy weight, overweight, or obese:

Although these are the general classifications that are used when talking about weight, there are a lot of other factors that really need to be considered when determining a teenagers overall health status. 

Based on the CDC growth charts classifications, a 13-year old boy who is 61 inches tall (5 foot 1 inch or 155 cm) would be considered a normal or healthy weight between 82 to 115 pounds (37 to 52 kg).

A 13-year old girl who is 61 inches tall (5 foot 1 inch or 155 cm) would be considered a normal or healthy weight between 81 to 118 pounds (37 to 54 kg). Other ages and heights would be classified differently.

What Factors Impact a Teenager’s Weight?

Eating and exercise can definitely impact a teenager’s weight, but there are other factors that play a role as well. It is important to look at all factors rather than just making assumptions based on one weight measurement and the classification that it falls under.

Factors That Impact Weight:

  • Eating habits
  • Exercise habits
  • Genetics
  • Social atmosphere
  • Distance to grocery store
  • Food budget
  • Time
  • Stress
  • Sleep habits
  • Medications
  • Hormones
  • Health conditions
  • etc.

Genetics is often an overlooked factor that influences weight. Social atmosphere and pressure from friends and classmates to look a certain way can be a big factor for teenagers too. Distance to a grocery store and food budget can also have an impact on weight and eating habits. 

Even if a teenager is underweight, the solution might not be as simple as just eating more or exercising less. 

Is It Unhealthy to Be Underweight?

Basing your health on one weight classification is not the best idea. Just because you are technically “underweight” does not automatically mean you are unhealthy. There are many other factors to be considered!

Your doctor should be monitoring your weight as you get older. Some people are just naturally smaller and don’t weigh very much! If this is you, your doctor can follow your trends over time to make sure you are still growing and developing properly, even if your weight stays lower than most other teenagers your age.

If your weight suddenly drops, or you start losing weight gradually as you are growing, this could be a sign that something is not right. Your doctor can assess your eating and exercise habits, but they can also run extra lab work to check for other underlying conditions such as hyperthyroidism, diabetes, and others. 

It is so important to be nourishing your body appropriately for what it needs. If your body is in a big growth phase, you need enough calories and nutrients to support it! Losing weight because you are not giving your body enough of what it needs can be dangerous and unhealthy.

13-year old’s are often very active and always on-the-go. It might feel challenging to keep up with high energy needs when you are just constantly burning through so much fuel! Working with a dietitian can be beneficial as they can help determine how much you should be eating to fuel all of your activities, and they can give you tips for how to eat enough!

Eating Disorders in Teens

There is no specific look or weight that is typical of all eating disorders. However, a decrease in weight, or changes in eating and exercise habits could be signs that there is some disordered eating going on. If you notice any of the following or other changes in behavior (this is not a comprehensive list), seek guidance from a doctor and dietitian on the best way to approach the situation and get help as needed.

  • Overexercising
  • Eating secretly
  • Hiding food
  • Avoiding certain foods or food groups
  • Showing fear or concern about eating 
  • Binging and/or purging behaviors
  • Feelings of guilt or shame associated with eating
  • Thinking about food constantly
  • Skipping meals 

When Should a 13-Year Old Gain Weight?

Rather than focusing solely on gaining weight, I like to encourage teenagers to focus on nourishing their body and giving it enough fuel to grow and support all the activities they are doing. This is a focus all teenagers can have- regardless of their weight!

A big part of that is developing a good relationship and connection to your body and its needs. These are some of the questions you might ask yourself:

  • Do I feel energized? 
  • Do I feel strong? 
  • Are you eating regularly throughout the day to provide a consistent energy source for your body? 
  • For females who have started their period: Am I getting a regular period?
  • Am I staying free from injury and sickness?
  • Am I sleeping well?

This isn’t an exhaustive list of questions, and answering “no” to these questions doesn’t automatically mean you need to gain weight, but it can help you do a self-evaluation and check in with your body. If you do answer “no” to any of these questions, or if something just seems off with your body, talk to your doctor and they can help you figure out what would be most beneficial to get you feeling your best!

Tips for Healthy Weight Gain for a 13-Year Old

If you do need to gain weight, how can you do it in a healthy way? You can probably guess that eating a bunch of fast food and milkshakes isn’t the best option, but what is? Here are some tips for weight gain from a registered dietitian:

  1. Eat regularly throughout the day and don’t go long periods of time without eating something. If you want to gain weight, you should be eating at least every 2-4 hours. Don’t skip meals. Try having a morning snack, afternoon snack, and evening snack, along with breakfast, lunch, and dinner. 
  1. Focus on including healthy, energy dense foods such as avocado, olive oil, nuts, nut butter, seeds, etc. These foods are higher in fat, but contain more unsaturated (good) fats. High fat foods are an easy way to get in more calories without having to consume a huge quantity of food.
  1. Drinking a lot of liquids at the same time you are eating might make you feel full sooner than you otherwise would. Maybe try leaving your liquids until the end of your meal. Try liquids with calories such as whole milk, 100% fruit juice, and smoothies.
  1. Don’t ignore hunger signals! And also don’t wait until you are absolutely ravenous to start eating. When you start to feel subtle hunger pangs, start planning something to eat!
  1. Keep snacks with you all the time. You can store extra snacks in your locker at school, in your backpack, in the car, at your parent’s office, etc! That way you never get stuck without having some food available. Great examples are fruit leather, trail mix, granola bars.
  1. Set reminders to check in with your body and eat regularly throughout the day. It can sometimes be tricky to remember to eat, especially if you have a busy schedule and are always thinking about other things. Some people find it helpful to put sticky note reminders in places they will see them regularly, and others set reminders on their phone!
  1. Put away distractions while you eat. Yep, that means put the phone away, turn off the TV, step away from homework, and actually sit down at the table to eat. This will make it easier to get in tune with your body and what it needs.
  1. Pay attention to how food makes your body feel. If you have more energy and generally feel better when you are eating more food, that can be really motivating to keep you going! 
  1. Weight gain can be a slow process, so don’t get discouraged if it doesn’t happen as quickly as you hope it will. Consistency is key!
  1. Work with a registered dietitian and your doctor to make sure everything stays on track and you are making appropriate progress in a healthy way! 

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How Can Parents Encourage Their Kids to Maintain a Healthy Weight?

Weight can be a tough topic, and knowing how to approach this difficult topic with kids is really important! Here are some tips for addressing weight, weight gain, and maintaining a healthy weight with teenagers:

  1. Don’t associate weight with worth. If your body weighs a certain amount or looks a certain way, that doesn’t say anything about you as a person. Many kids and teenagers internalize body comments in this very impressionable phase of life.
  1. Avoid talking about weight as much as possible. Remember, the amount your weight is not the most important indicator of your health status. Focus more on nourishing your body and fueling appropriately. 
  1. Develop a relationship with your child’s doctor, and make sure they are on board for talking about weight in a neutral way and focusing more on healthy habits. Your doctor can be a great resource, especially in determining if there are other medical issues that need to be addressed.
  1. Keep snacks available for your teenager when you are out and about. Help them remember to bring snacks when they are going out with others and help them pack  healthy, balanced snacks that will give them energy and nourishment.
  1. Gather your family to sit and eat together at mealtimes as much as possible to make eating occasions a positive experience. Keep stressful conversations away from the table and build good memories!
  1. Set a good example of healthy eating and nourishing your body by eating a variety of foods, and not labeling foods as “good” or “bad.” Talk positively about your own body and avoid making comments on your own weight or the weight of others. Instead, talk about how foods make you feel!
  1. Encourage your child to check in with their body regularly throughout the day. Remind them to think about when the last time they ate was and to evaluate what their body needs to stay energized.


As teenagers focus on fueling their body appropriately based on their individual needs, they will reach a weight that is healthy for them. Remember to keep your doctor on board with what is going on so that they can make sure things stay on track and deal with any problems that arise in the process!

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Academy of Nutrition and Dietetics. BMI for Children and Adolescents. Eatrightpro.org. Published August 26, 2015.

Katherine Harmer, RDN

I'm a Registered Dietitian Nutritionist with a love for coaching others to success in their health goals, especially teenage athletes. Tennis was my sport of choice in high school. Now I'm a little bit older, a little bit smarter, and a little bit worse at tennis.

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