What Is BMI?

Let’s start from the beginning. BMI, or Body Mass Index, is an estimate of body fat, calculated using a person’s height and weight. To determine BMI, all you need is a scale, a tape measure, and an online BMI percentile calculator like this one. Starting from age 20, for both men and women, a BMI under 18.5 is considered underweight, 18.5-24.9 is normal weight, 25-29.9 is overweight, and 30+ is obese. It’s pretty straightforward.

How To Use BMI for a Child Between 2 through 19-years-old

Straight BMI numbers can’t be used as accurate health indicators for kids because the range of “normal” BMI fluctuates during this time. Children often go through periods of “chubbiness” before hitting growth spurts that thin them out as their height increases. Because of this natural childhood growth pattern, a child’s age and gender are taken into account.

Health professionals use BMI percentiles between the ages of 2 through 19 years old. A child’s BMI is compared to the BMI of other children her same age and gender. If she falls between the 5th and 85th percentiles (meaning, for every 100 kids, only 5 have lower BMIs and only 15 have higher), she is considered within the “normal” range. Use the handy Child and Teen BMI Calculator below to find your child’s BMI and where it falls in percentile, given your child’s age and gender.

What do these results mean? Health professionals divide the percentiles into four classifications:

• Underweight – When a child’s BMI is below the 5th percentile (or below 95% of other children his age, height, and gender), he falls into the classification of underweight. A child who is underweight may need help ensuring he’s fueling his body with enough nutrients to power his development.
• Healthy weight – When a child’s BMI is between the 5th and 85th percentiles for his age and gender, he’s considered to be at a healthy weight.
• Overweight – When a child has a BMI at or above the 85th percentile but less than the 95th percentile, the child is categorized as overweight. Parents can check in with their pediatrician to discuss their child’s growth trajectory and if any lifestyle changes should be made.
• Obese – A BMI above the 95th percentile is defined as obese. In this case, a child will have a greater BMI than 95% of other children his age and gender. At this level, the proportion of body fat to a child’s height may cause developmental problems such as delayed growth, joint, muscle, and bone discomfort, as well as issues with the heart and increased risk for serious illnesses.

Drawbacks of BMI

Please note that BMI isn’t a perfect indicator of a child’s health because while it correlates with body fat, it’s not a direct measure. BMI can’t differentiate if the weight comes from body fat or muscle (which is denser than fat and can increase a child’s weight in a perfectly healthy way). Additionally, it can’t measure other parts of body composition that could affect weight, such as bone density. Essentially, that means a child could be outside of “normal” range and still be perfectly healthy!

With this grain of salt in mind, parents can use BMI percentiles as an estimate of their child’s developmental progress. If your child’s BMI is outside the normal percentile ranges, this might be a signal to talk to your pediatrician about whether lifestyle or diet changes need to be made.

Obesity, Health Risks, and BMI

Childhood obesity is a health risk that occurs when your child is above the 95th percentile in her BMI charts.

Obesity can cause potential health complications for children both in the short term and farther down the road. An overweight child is more likely to be obese as an adult, which can lead to an increased risk of diabetes, heart disease, and more serious illnesses later in life.

Some of the immediate consequences of childhood obesity may include:

• Breathing problems or sleep apnea
• High blood pressure and high cholesterol
• Type 2 diabetes
• Chronic joint pain
• Psychological and mental health issues

If your child’s BMI is raising any red flags, you may want to discuss with your pediatrician and develop plans that support healthy growth.

A Word About Fat

With all this talk of weight, it may seem like fat is the other f-word. But truly, fat has a worse reputation than it deserves. Fat does all kinds of amazing things for your child’s body, including:

• Acts as a source of energy for your rambunctious little one
• Helps your child’s body absorb important vitamins like vitamins A, D, E, and K
• Supports hormone production, which maintains important aspects of the body like muscle, bone, and brain development, sleep, mood regulation, metabolism function, and more.
• Plays a critical role in a child’s brain development.

It’s no wonder experts recommend that 25-35% of a child’s diet from ages 2-18 should be fat! As you learn more about healthy weights and BMI percentiles for your little one, keep in mind that a zero-fat lifestyle isn’t the same as a healthy lifestyle.

How To Help Your Child Maintain a Healthy BMI

Worrying about your child’s BMI can be tough. You want to make sure your little one grows up healthy and happy without having a strenuous relationship with food or her body—and you absolutely can. There are things you can do at any age to help support your child’s long-term wellness, such as:

• Learn more about your child’s nutritional and dietary needs – Diets full of nutrient-rich fruits and veggies, lean proteins, whole grains, and the occasional treat can give your child’s body the fuel it requires to function. On the other hand, diets high in calories and low in nutrients can lead to an excessive intake of calories, leading to weight gain. Talk to your child’s healthcare provider about your child’s calorie and nutrient needs at this particular stage in her life.
• Make time for playtime – Your child loves to play—whether that’s swinging on the monkey bars or chasing a sibling or pet around the house. In fact, play and exercise are essential to a child’s development. An inactive lifestyle in which your child isn’t burning enough calories during playtime means an imbalance in calories taken in and calories being used—a.k.a. a recipe for an unhealthy BMI. Encouraging playful habits can help ensure your child’s body grows up strong and healthy.
  • What is baby BMI and does it matter? Children ages 0-3 need at least 30 minutes of play a day. For a baby, that can be just a little bit of crawl-practice, and for toddlers, that can be time spent on the playground!
  • Health experts recommend that kids ages 3-5 should spend at least 3 hours a day engaged in a variety of activities. Essentially, they shouldn’t be inactive for long periods of time. Keep your little one moving with games, toys, and of course, playing with family!
  • Kids from ages 6-17 need about an hour of moderate-to-vigorous physical activity every day. The CDC recommends kids in this age range spend three days a week on aerobic exercise (like playing soccer or swimming) and three days a week doing strengthening exercises like (push-ups or climbing).

• Pay attention to your child’s sleep health – Check in with your little one’s sleep needs. By making sure your little one is getting plenty of shuteye, you can also make sure that your child’s body is at its healthiest. Studies show a link between sleep deprivation and weight gain, particularly in children who need more sleep than adults as they grow.
• Keep healthy fats on the plate (and enjoy not-as-good fats in moderation) – Fat plays an important role in your child’s growing body. But like all good things in life, everything is better in moderation. Try choosing healthy sources of fat (foods with monounsaturated or polyunsaturated fats like avocados, salmon, extra virgin olive oil, chia seeds, and eggs) that work with the body. That way, you can ensure that the fats in your child’s diet are put to good use in his health and development.

Understanding and maintaining a healthy BMI for children aged 2 through 19, involves considering the child’s age, gender, and natural growth patterns, rather than relying on a straight BMI value. While BMI isn’t a perfect health indicator, it can serve as an estimate of a child’s developmental progress and act as a potential signal for discussing lifestyle or diet changes with a pediatrician, ensuring a balanced approach to nutrition, exercise, and rest that promotes long-term wellness.