What Is BMI, and How Much Does It Really Matter?

Body mass index (BMI) is a measurement that many people worry about, but don't really understand. After its creation in the early 19th century, the body mass index (BMI) formula spread quickly, likely because it is fairly simple and requires no special equipment to give you a marker of your body composition. In fact, many doctors and health organizations — including the Centers for Disease Control and Prevention — still rely heavily on BMI when assessing an individual's health. With this kind of prevalence and accessibility, it's worth asking: What is BMI, and how accurate is it?

What Is BMI, and How Is It Calculated?

The BMI formula is relatively simple and was designed over 200 years ago with the idea that most people could calculate it in their head. Essentially, the formula is a ratio of your weight to your height. Because the BMI was invented by a Belgian mathematician, it was intended to work with the metric system. Therefore, American users must also apply a conversion factor. There are many BMI calculators available online, and they all use the same formula: weight (in pounds) x 703/[height²(in inches)] = BMI.

Once you've completed the math and have your BMI, you compare the result to a chart that classifies your body composition into one of four categories. According to the CDC, those ranges are:

• Below 18.5 = Underweight.
• 18.5 to 24.9 = Ideal.
• 25.0 to 29.9 = Overweight.
• 30.0 and above = Obese.

The same formula is used regardless of age or gender, though the results are usually interpreted differently in children and teens than they would be in adults.

What BMI Does and Doesn't Tell You

Clearly, the appeal of using BMI is that it can be calculated by anyone with no special equipment or training. Unfortunately, its simplicity may be a limiting factor. In fact, the creator of the BMI formula, Lambert Adolphe Jacques Quetelet, specifically said that the measure should not be used to measure the health of an individual. Instead, it was intended to provide a largely statistical view of the population, according to NPR's math consultant Keith Devlin.

One of the major downfalls of the BMI formula is that it doesn't factor in aspects such as age and gender that can have a large bearing on body composition. BMI also ignores waist circumference, another indicator of body fat and a risk factor for heart disease and diabetes. The formula is particularly inaccurate when it comes to athletes or other individuals who have considerable muscle mass and low body fat, since it does not recognize these aspects of health.

Nonetheless, BMI does provide a fast and convenient reference point to get a statistical measure of health. For a more accurate body composition test, consider body fat testing with calipers. Another easy, alternative method is to simply measure the circumference of your waist: The National Institutes of Health states that a healthy waist circumference for men is 40 inches or less, while the limit is 35 inches or less for women.

Once you understand your body's composition, you and your doctor can discuss how to monitor your health or make lifestyle changes. By keeping an eye on your body, you can minimize certain controllable risk factors and make sure you are maintaining a healthy weight.