What Is BMI?
Body mass index (BMI) is a measurement that helps us describe whether a person is underweight, has a healthy weight, is overweight and is also used in the diagnosis of obesity. To be exact, BMI is calculated by dividing a person’s weight (in kilograms) by the square of their height (in meters).
BMI is not a direct measure of someone’s body fat. Rather, it measures the body weight relative to their height. This does give health care professionals a quick, easy and accurate enough indication of body fat and can predict certain diseases.
How Is It Used?
BMI is used as a quick and easy screening tool for a person’s weight category and for diseases such as obesity and diabetes. It is not used by itself to make a diagnosis in these cases but is a helpful tool and can be an accurate predictor.
To calculate your BMI, you will need your weight and height. The formula for BMI is a person’s weight divided by the square of their height, all using metric units of kilograms and meters. If you are using the imperial units (pounds and inches), you will need to add another element to the formula to get the measurement.
In metric units
Formula: weight (kg) / [height (m)]2
In the imperial system (used in the U.S.)
Formula: weight (lb) / [height (in)] 2 x 703
The formula shows that the taller you are, the lower your BMI will be. Conversely, the more you weigh, the higher your BMI. The calculation is set up in this way as people who are taller will tend to weigh more as well (and vice versa), so the formula takes that into account.
What Do the Results Mean?
You can see a chart that summarizes how the BMI results can be generally interpreted on the Centers for Disease Control and Prevention (CDC) website.
A lean or underweight person has a BMI of 18.5 or less. At a BMI of 25, a person is considered to be overweight. The higher the BMI, the higher the health risk. The threshold for obesity is a BMI of 30 or higher. Note that there are cases, however, such as for muscular athletes, where the BMI is in a higher category (such as obesity) but the person does not have excess body fat. This occurs because the BMI calculation uses a person’s total body weight, which considers muscle mass and other tissues in addition to body fat.
Differences in BMI Results for Children and Teens
Children and teens are still growing and their amount of body fat changes with age. Even though the BMI is calculated using the same formula described earlier, the results are not interpreted in the same way. The CDC developed a BMI-for-age growth chart that takes into account age, sex and other factors to show BMI as a percentile ranking for children and teens. For example, for 2- to 19-year old children, obesity is defined as a BMI at or above the 95th percentile (greater than that of 95%) of children of the same age and sex. Visit the CDC website for more information.