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Your Healthy Weight: Body Mass Index

By Jennifer Ashton, M.D., Ob-Gyn with Christine Larson
WebMD Feature by “The Body Scoop for Girls”

 

With more body types than car models out there, it’s not always obvious what a healthy weight might be. So doctors look at the ratio of your height and weight, then compare it to other people like you to see if you’re in the healthy zone.

BMI Worksheet

In other words, your BMI equals your weight in pounds divided by your height in inches squared, all multiplied by 703.

Healthy BMI for Children and Teens

Weight Category

 Percentile

Underweight

Less than 5th percentile (fewer than 5 percent of teens your age and height weigh less than you do)

Healthy weight

5th to 85th percentile

Overweight

85th to 94th percentile (15 percent or fewer teens your age weigh more than you do)

Obese (so overweight that serious health problems may result)

95th percentile or higher

 

This system is called the Body Mass Index. It’s an easy calculation where you divide your weight in kilograms by your height in meters squared. (OK, it sounds complicated, but it’s easy to do. . If you don’t feel like doing the math, go to www.cdc.gov and search on “BMI for teens” to get an online calculator.)

Typically, the lower your BMI, the less body fat you have. While you need a certain amount of body fat to be healthy, generally speaking you want it to be on the lower side. For adults (people twenty and older), a BMI of 18.5 to 24.9 is normal and healthy. Lower is underweight. Higher is overweight. A BMI over 30 is obese (that is, the person is so overweight that they may have serious health problems).

For teens the calculation isn’t quite so straightforward, since teen bodies are still growing and are even more different from each other than adult bodies are. Instead of just looking at your BMI number, your doctor will compare it to the BMI of other people your age to determine what’s healthy and normal. If you’re below the 85th percentile (that is, if 15 percent or more of all teens your age and height weigh more than you do), you’re considered to have a healthy weight. If you’re in the top 5 percent, you’re obese; if you’re in the bottom 5 percent, you’re underweight. The CDC site on BMI for teens and children can show you where your normal range is.

For example, if you’re sixteen years old, five feet six inches tall, and weigh 120 pounds, your BMI is 19.4. At the CDC Web site, you’ll find that puts you in the 35th percentile for girls your age—healthy!

Once you know your BMI, you may want to check it every year or so to see where you fall compared to last year. This is one area of life where you really want to strive for the middle 85th percentile; being at the 95 percentile for BMI can be just as unhealthy as being at the 5 percentile for BMI.

Your BMI gives you a rough idea of where you stand weightwise, but it’s far from perfect. It tells you how you compare to everybody else your age and size, but it doesn’t account for things like whether you’re athletic or whether you have an unusually high or low BMR. Girls with more muscle may have a falsely high BMI number, since muscle weighs more than fat. In fact, when professional athletes measure their BMI, their results often suggest that they’re obese, which couldn’t be less true!

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