Body Mass Index (BMI)

Young men's version of this guide
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BMI is used to estimate a person’s body fat based on their height and weight. Everyone needs some body fat, but having too much body fat can put a person at risk for health problems such as high blood pressure and diabetes. Having too little body fat is also unhealthy.

How is BMI measured?

BMI is a mathematical calculation of your weight (in pounds) times 703 divided by your height (in inches) squared. You can figure out your BMI on your own using this formula: BMI = Weight (in pounds) x 703 divided by Height (in inches) squared.

For example, if you weigh 130 pounds and are 5 feet, 6 inches tall, your BMI = 21.

In this example, BMI = 130 x 703 divided by 66 (squared) = 91390 divided by 4356 = 21.

You can use online calculators to figure out your BMI: Teens and Adults (20+). But it’s best to ask your health care provider or school nurse for this information to make sure your height and weight are measured correctly.

What does my BMI mean?

If you’re under the age of 20, once your health care provider or school nurse calculates your BMI, they must use a special chart to convert this number into a percentile. Your percentile tells you how your BMI compares with other teens your age and gender. This is based on a growth chart. Your health care provider can tell you if your BMI percentile falls into the underweight, normal weight, overweight or obese category.

BMI percentile:Category:
less than 3rd percentileUnderweight
between 5th and 85th percentileNormal Weight
between 85th and 95th percentileOverweight
greater than 95th percentileObese

No matter what your BMI percentile is, it’s important to ask if it has changed over the last couple of years. Although BMI can change when you’re growing or going through puberty, teens should ideally stay at about the same BMI percentile during their teen years. If your BMI percentile changes a lot either up or down, you should talk to your health care provider about possible reasons for the change. For people 20 years and older, BMI is compared against all other adults; healthy ranges do not differ by age or by gender and are no longer given in percentiles.

What is a BMI report card and what is it used for?

Some schools in the United States are checking weights and heights of students at school, calculating BMI, and sending this information home to parents as a BMI report card. The report cards are used to identify teens who are underweight, overweight, or obese. Teens who fall in any of these groups may be at a higher risk for weight-related health problems.

What if my BMI is high?

Talk to your health care provider about whether you should be concerned about your BMI and about healthy ways to manage your weight. Keep in mind that BMI is just an estimate of body fat. Most teens who have a BMI in the overweight or obese category  have a high amount of body fat, but there are some who don’t. This is true for muscular teens (especially those who exercise or are very athletic and toned). Their weight may be higher because of extra “muscle mass” and not because of extra body fat.

If your BMI is falls in the underweight, overweight, or obese category, you should see your health care provider. He/she can check to see if you are at risk for getting any serious health problems and help you work on a healthy weight management plan, if needed.
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