The body mass index, or BMI, is commonly used to determine your amount of body fat. BMI is calculated by using your height and weight. Your doctor calculates your BMI to determine your risk of obesity and related diseases. While BMI is a simple, inexpensive formula for determining your body fat, it does have some limitations.
Calculation
Calculate your BMI by multiplying your weight in pounds by 703. Divide that number by your height in inches squared. For example, if you are 5'2" and weigh 110 lbs., the calculation would be 110 x 703 / 3844. Your BMI equals 20.12. You will get the same number if you divide your weight in pounds by height in inches squared and then multiply that number by 703.
BMI Categories
Your BMI will fall into a certain category. A BMI below 18.5 means you are underweight. Being underweight may put you at risk for malnutrition. An ideal BMI is between 18.5 and 24.9. You are considered overweight if your BMI is over 25.0 and obese if it is above 30.0. Having a BMI above 25 may increase your risk for hypertension, high cholesterol, coronary heart disease, stroke and some cancers, according to the Centers for Disease Control and Prevention.
Body Size
The BMI only uses your weight and height to determine your amount of body fat. Body size is not used in determining your BMI, reports the American Heart Association. If you have a large frame or are big-boned, you may have a high BMI but not necessarily a high percentage of body fat. Weight is not the only factor to determine your risk of chronic diseases. Talk with your health care provider if you are concerned about your weight or your risk of weight-related diseases.
Limitations of BMI
The body mass index is not a direct measurement of body fat. Body fat can be measured directly by using under-water weighing or skin-fold measurements. Trained athletes may have a high BMI but a low percentage of body fat. Muscle mass weighs more than fat, and individuals with a lot of muscle may have a high BMI. Women naturally have a higher percentage of body fat. A man and a woman of similar sizes may have the same BMI, but the woman will have more body fat. Seniors also have a tendency to have a higher percentage of body fat. A young woman and an older woman may have the same BMI, but the senior woman will most likely have more body fat.
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