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How to Use Your Body Mass Index

By

Updated May 16, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

scale

Find your Body Mass Index to know if you are at a healthy weight.

Tara Bartal

The Body Mass Index (BMI) combines your weight and your height into a score that helps you determine if you are underweight, at a healthy weight, overweight, or obese.

Since a typical scale only measures your total weight, it helps to have more information to determine if that weight is healthy or unhealthy. A person who is six feet tall and weighs 198 pounds is probably going to have a smaller amount of body fat than a person who is five feet tall and 198 pounds. 

BMI is calculated with the following formula:

weight (lb) / [height (in)]2 x 703

or in metric:

weight (kg) / [height (m)]2

You can use a BMI calculator to do the math for you.

You can compare your BMI to this table to help you determine whether you're at a healthy weight.

  • Underweight = less than 18.5
  • Normal weight = 18.5-24.9
  • Overweight = 25-29.9
  • Obese = 30 or greater

If you are planning to lose or gain weight, you can use your BMI to monitor your progress. It's important to know that your BMI is not the same as your body fat percentage, which is a different number and doesn't correspond to these charts.

People who have a BMI in the overweight or obese ranges may be at a higher risk for cardiovascular disease, diabetes, arthritis, and some forms of cancer. It's important to see your health care provider, to evaluate your health.

Not for Everyone

The BMI isn't perfect because it's an indirect measurement of fat, so really doesn't differentiate pounds of fat from pounds of muscle and bone. It doesn't work well for very muscular people or for people who have lost a lot of muscle mass.

For example, an elite athlete with a very small amount of body fat will still have a high BMI, and an elderly person may have a lower BMI because they have less muscle mass. In these cases, a better method of measurement is the body fat percentage.

Additional Ways to Monitor Body Composition

You'll get a clearer picture of your body composition if you use two or three different measurements. Weigh yourself on a regular bathroom scale, or use a fancier scale to track your body fat percentage. Use a tape measure to measure your waist.

Source: 

Centers for Disease Control and Prevention. "About BMI for Adults." http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html

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