Know Your Child's or Teen's Body Mass Index - BMI Calculator

By Jo Ellen Shield, MED RD LD
Mary Mullen, MS, RD
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Want to know how your child's weight measures up? It's tricky to tell whether a child or teen is overweight. Since kids are still growing, they need to gain weight and they are supposed to outgrow their clothes. Te best way to determine whether your child's weight is healthy is for you to measure and keep track of his or her body mass index (BMI). That's what pediatricians do, and you can do it, too.

For years, pediatricians have used height and weight measurements to compare your child's growth to other boys and girls the same age. Now they have another tool: BMI, a number calculated from your child's height and weight that can be used to estimate body fat. BMI does not measure body fat directly, but many studies have shown that it is a reliable indicator of body fatness and disease risk for most kids. Plus, it's an inexpensive and simple method for determining whether a child's or teen's weight is healthy. In fact, it's so simple you can calculate your own son's or daughter's BMI with an ordinary calculator or by using an online tool. Just keep in mind that to calculate BMI accurately, you must start by obtaining an accurate height and weight.

How to Calculate BMI

You don't need to be a math whiz to calculate BMI. Just grab your calculator and use on the two basic BMI formulas. It doesn't matter which formula you use since they both end up with the same BMI — so you make the call. After you have done the math, round off the BMI to the nearest whole number.

  • Formula 1: BMI = Weight (pounds) / Height (inches) / Height (inches) x 703
  • Formula 2: BMI = Weight (kilograms) / Height (centimeters) / Height (centimeters) x 10,000

Let's calculate BMI for a boy named Peter. If Peter weighs 129 pounds (59 kilograms) and is 63 inches (160 centimeters) tall, his BMI is 23.

  • Formula 1: BMI = 120 / 63 / 63 x 703 = 23
  • Formula 2: BMI =59 / 160 / 160 x 10,000 = 23

Interpreting BMI

When BMI is used to evaluate weight in adults, the BMI number alone is sufficient to place the individual in a weight category (underweight, normal, overweight, or obese). However, we can't use an adult BMI chart for kids because their body fatness changes as they grow. Also, girls and boys differ in body fatness, especially as they mature, so we need to use gender-specific criteria for evaluating BMI in kids.

To address these differences between adults and kids, and between girls and boys, health care professionals use BMI-for-age-percentile growth charts to interpret BMI in boys and girls ages 2 to 20 years. These graphs list ages along the y axis (vertically) and BMI along the x axis (horizontally), and show curves for different BMI percentiles. To use a BMI growth chart, you plot (mark) the point on the graph where your child's age and BMI cross, and then take note of either the percentile curve the point lands on or the curves on either side of this point. The percentiles of those curves become the key to understanding whether the BMI is in a healthy range. You can use this online chart from the Centers for Disease Control and Prevention.

How do you interpret your child's or teen's BMI percentile? Based on pediatric body fat research, medical experts have come up with the following weight categories for children and teens:

  • Underweight: BMI is less than the 5th percentile.
  • Healthy weight: BMI is between the 5th and 84th percentiles.
  • Overweight: BMI is between the 85th and 94th percentiles.
  • Obese: BMI is equal to or greater than the 95th percentile.

It's important to point out that "overweight" and "obese" are medical terms and should not be used in front of kids. It's best to talk to your child or teen about being a healthy weight.

Tracking BMI Over Time

A single BMI calculation does not tell your child's whole weight story. As kids grow, their body fat changes, which means their BMI will change, too. That is why it's important to track BMI over time.

A final word of caution: BMI is the gold standard for screening potential weight problems — it cannot be used to make a final diagnosis. For example: If your child or teen is very athletic and muscular, his or her BMI may be high because of extra muscle mass, not body fat (muscle weight more than fat).

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