Body mass index, or BMI, is a measure used to approximate your amount of body fat. The equation is readily available via online calculators so you can roughly estimate whether you're overweight, underweight or obese, or you can easily calculate your own BMI using a basic equation. But make sure you consider more than just BMI when assessing your health, since it's not a perfect measure for everyone.
Body Mass Index Equation
To figure your body mass index, divide your weight in pounds by your height in inches squared; then multiply the result by a conversion factor of 703. The formula is:
BMI = weight in pounds / [height in inches x height in inches] x 703.
A person who weighs 150 pounds and stands 5 feet, 6 inches thus has a BMI of 24.
Alternatively, figure BMI using metric system measurements of kilograms and meters. No conversion factor is needed for this equation:
BMI = weight in kilograms / [height in meters x height in meters].
In the example, the person weighs 68 kilograms and stands 1.68 meters with a BMI of about 24. Figure your weight in kilograms by dividing your weight in pounds by 2.2; figure height in meters by dividing your height in inches by 39.37.
The number the BMI equation yields gives you an idea of the appropriateness of your body size. If the result is 18.5 or below, you're considered underweight. If the number measures 18.6 to 24.9, you're in the normal range. Results of 25 to 29.9 qualifies as overweight and 30 or more indicates obesity.
BMI isn't enough to diagnose you as having conditions associated with being overweight or obese, but it does alert your medical provider that you're at risk for health issues such as high blood pressure, high cholesterol, type 2 diabetes and heart disease.
Advantages of BMI
Figuring BMI with a pencil, paper and calculator is simple, noninvasive and inexpensive; you can do it at home using readily available measurements. More precise measures of body fat require special equipment, such as scales or calipers, and special training, in the case of hydrostatic weighing or DEXA scans.
BMI is usually accurate at estimating obesity for people who have a very high BMI. If you're at a healthy BMI, though, the calculation isn't quite as accurate, so you might consider investing in body composition testing to ensure you're not carrying too much fat.
Shortcomings of BMI
While BMI is generally useful, it's not the perfect measure of obesity or disease risk. BMI measures your entire body weight, and it doesn't take into account whether that weight comes from fat or lean mass. People with a large amount of muscle mass may end up with a high BMI, but in fact be perfectly healthy because they carry very little excess fat.
Conversely, BMI can underestimate health risk in people who are at a "normal" weight, but carry more fat than is healthy. A body fat percentage of 20 percent or greater on a man or 30 percent or greater on a woman could put you at risk of the health conditions associated with obesity, such as type 2 diabetes and heart disease. Sedentary people and older adults, who've naturally lost muscle mass and bone density, may fall into the category of normal-weight obesity.
Your doctor can evaluate your appearance, family history and lifestyle habits to determine whether your BMI results are an indication of concern.