Your doctor may use your body mass index, or BMI, as a way to generally assess your weight as well as your risk of weight-related health problems, including high blood pressure, heart disease, type-2 diabetes and certain types of cancer. BMI isn’t a direct measure of excess body fat because the calculation for BMI doesn’t discern between weight that comes from fat and weight that comes from muscle or bone. Although it isn't a perfect measure for everyone, BMI is considered a fairly accurate indication of whether or not you’re at a normal, healthy body weight.
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Normal BMI Range
BMI evaluates body weight relative to height. It’s calculated by dividing your weight in pounds by your height in inches, squared, and then multiplying the result by a conversion factor of 703:
BMI = weight in pounds / (height in inches x height in inches) x 703
For example, a person who weighs 130 pounds and is 5 feet 5 inches tall would calculate BMI like this: BMI = 130 / (65 x 65) x 703 = 21.6.
If you don't want to do the math, you can compute your BMI using an online calculator by simply entering your weight and height.
A normal BMI is defined as one that falls anywhere in the range of 18.5 to 24.9, so the person in the previous example has a normal BMI. There are three other BMI categories, though. A BMI score below 18.5 is considered underweight, while one that falls in the range of 25 to 29.9 qualifies as overweight, and a BMI score above 30 points to obesity.
At its core, BMI is a mathematical relationship between weight and height, and what's considered normal under BMI parameters is wide-ranging. Because the BMI formula presumes that weight increases at a certain rate as height increases, shorter people have smaller body weight ranges for a normal BMI -- and other BMI categories -- than do taller people.
For example, an adult who is 5 feet tall would have a normal BMI when weighing anywhere from 97 to 127 pounds, according to tables from the National Institutes of Health. Someone who is 5 feet 6 inches tall would have a normal BMI when weighing anywhere from 118 to 154 pounds, while an adult who is 6 feet tall would have to weigh between 140 and 183 pounds to have a normal BMI.
BMI and Health
Although having a normal BMI doesn’t necessarily mean you’re healthy, someone with a normal BMI has, on average, a lower risk of developing certain chronic diseases than someone with a higher BMI. Better health is strongly associated with having a normal BMI, according to a 2006 study published in the International Journal of Obesity. The large study compared the BMI ranges of approximately 11,400 healthy men and women. The study found that healthy men and women -- where health levels are defined by blood pressure, blood lipid and blood glucose readings, overall medical history, and health-affecting behaviors like exercise and smoking -- are far less likely to have a BMI that qualifies them as overweight or obese, particularly as health levels increase. Statistical data revealed that the median BMI for healthy men in the study was 24.5, while the median BMI for healthy women was 21.5.
Limitations of BMI
BMI is a screening tool, not a diagnostic tool, so while it's used as an indirect measure of excess fat to assess your potential health risks, it isn’t necessarily an accurate predictor of those risks. One reason for this is that BMI doesn’t take fat distribution into account. So if your weight is normal but you have too much belly fat, your BMI number erroneously classifies you as healthy. The truth is, if you carry a disproportionate amount of fat around your midsection, you have a higher risk of developing type-2 diabetes, heart disease and other health problems -- even if your BMI is considered normal.
BMI can improperly classify people with higher weights, too. It's fairly ineffective when used to screen athletes, primarily because it doesn’t differentiate between fatty tissue and lean tissue. By BMI standards, it’s not unusual for fit, muscular athletes to qualify as overweight and bodybuilders with very little fat to qualify as obese, because both groups weigh more due to an abundance of muscle tissue.