BMI and Kids: Should You Care About Your Child's BMI?

Maintaining a healthy weight is important at any age. Unfortunately, many children and teens struggle with their weight. Helping your kids get to and stay at a weight that is appropriate for their individual considerations benefits their overall health and well-being now and in the future. 

Body mass index (BMI) is a tool doctors use to compare weight in relation to height, which gives a quick estimate of body fat. It also determines if your child is underweight, overweight, or at the right weight for their height and age. 

Though your child’s BMI is part of their annual physical exam, it’s not the only measure doctors use to assess weight or health. In fact, the American Academy of Pediatrics (AAP) says that BMI is far from being the perfect tool for assessing health or weight in children and teens.

Many factors influence your child’s weight, including genetics, age, metabolism, height, physical activity, and environment. BMI is only one measuring tool and it only takes into account your child’s weight, height, and age.

Read on to learn more about the BMI, how accurate it is for kids, and how to talk to your pediatrician about it.

What Is BMI?

BMI is a simple tool that compares weight to height to determine weight categories: underweight, normal weight, overweight, or obesity. These categories are the same for children, teens, and adults. 

However, the BMI numbers that categorize an adult’s weight don’t work for children and teens. There are many reasons why this doesn't work, but mainly, kids are still growing and there are differences in body fat between boys and girls.

Instead of a range of numbers, BMI for children and teens is categorized as a percentile based on weight, height, age, and sex.

BMI-for-age percentile weight categories are as follows:

• Underweight: BMI less than 5th percentile
• Normal weight: BMI ranging from 5th percentile to 84th percentile
• Overweight: BMI ranging from 85th percentile to 94th percentile
• Obesity: BMI at or greater than the 95th percentile

Your child’s health care provider uses BMI to get a general sense of your child’s nutritional, activity, and health needs.

How Accurate Is BMI for Kids?

The BMI mathematical formula was originally created in the 1840s as a tool to measure the dimensions of the average man. It remained unused for more than 100 years. In the 1970s, epidemiologists used BMI as a standardized tool for measuring body fat in large populations and then as a tool to define obesity in studies and large populations.

BMI was never meant to serve as a tool to assess individual health, body fat, or weight status. Additionally, BMI doesn’t measure body fat directly, it estimates body fat status.

In children and teens, BMI is even less accurate at assessing body fat. Your child’s BMI isn’t a measurement of their body composition or the percentage of how much of their weight comes from muscle and fat. 

A 2017 study published in BMC Pediatrics found BMI to be a poor tool for measuring total fat mass and percentage of body fat in children 9 and younger. In children older than 9, the researchers found BMI to be a good indicator of total fat mass, but not so good at measuring body fat percentage.

When it comes to assessing health and risk of disease, body fat percentage provides a more accurate picture than total fat mass. Body fat percentage is how much of your total weight comes from fat. If you have a high percentage of body fat, no matter how much you weigh or what BMI weight category you fall into, you’re at greater risk of developing health problems like heart disease.

BMI is a screening tool, not a diagnostic test. A child with a BMI percentile that falls outside of the normal weight percentile range shouldn't set off alarm bells. Using BMI in combination with other parameters, such as nutrition and physical activity, may provide a more well-rounded picture of health and weight status.

How to Talk to Your Child's Pediatrician About BMI

If you have concerns about your child’s weight, you may turn to online BMI calculators or health-tracking apps to figure out where your child falls so you can take action. Though BMI is the go-to tool for health practitioners and health apps, it’s not the only way to measure health, especially in children and teens.

Before you start counting calories, talk to your child’s pediatrician about BMI and what it means for your child. There’s no “ideal weight,” especially for growing children and teens. And height and weight aren’t the only factors that influence your child’s health.

You may see significant changes in your child’s weight and height when they hit puberty. Adequate nutrition is essential during this period of rapid growth. However, if your child’s BMI is less than the 5th percentile or greater than the 85th percentile, it’s time to talk to the pediatrician for guidance.

BMIs that fall out of the normal weight percentile range are signs of potential health problems. Children and teens with a BMI greater than the 95th percentile are at risk of developing health problems like high blood pressure or high cholesterol. Kids who fall out of the “ideal weight” range may also experience teasing and bullying, which may affect self-esteem.

The Centers for Disease Control and Prevention (CDC) says children and teens with BMIs that fall out of the normal range need further evaluations and testing. This might include tests that assess body composition and body fat percentage, such as skinfold thickness measurements. 

Risk of Focusing on BMI

The US Preventive Services Task Force (USPSTF) recommends health care providers use BMI to screen for obesity in children and teens. However, focusing too much on BMI and weight may put pressure on your kids that leads to disordered eating. 

Children and teens may find weight talk hurtful. This includes parents talking about their own weight or remarking about their child’s weight, even when said with good intentions. Weight talk is a risk factor for eating disorders.

Instead of BMI and weight, focus on lifestyle and the importance of balance. Kids need good nutrition for growth and development. Providing nutritious meals benefits your child’s growth and teaches them how to make better food choices now and in the future.

Eating together as a family also improves nutrition. Families who eat together consume more fruits, vegetables, and whole grains. Serving as a role model for your child, gives them the ability to stay in control of their own body and health, or body autonomy.

A Word From Verywell

BMI is the go-to tool for weight-status screening for kids, teens, and adults. Though your child’s BMI is an important part of their annual wellness exam and something you should discuss with the pediatrician, it’s not the final marker of health and wellness. 

Instead of BMI, focus on your child’s habits. Your child’s nutrition and activity habits are better markers of health than their BMI weight category. Help your child create healthy habits by providing balanced meals, offering nutritious snacks, and creating opportunities for physical activity.