BMI is calculated via the following formula:

Thus, it is merely a factor of your weight and height. 703 is a conversion factor from imperial to metric units as the formula was originally derived by a Belgium statistician early in the nineteenth century. It is incredibly accurate at predicting the overall state of health of a population, but for individuals it is terrible. Now I will say that the CDC and most physicians use this measurement as a general rule of thumb, but it is still mind boggling trying to figure out why.

What is wrong with it? Scientifically, it makes no sense. It makes no account for body composition or waist size, the primary indicators of obesity level. Using myself as an example, if I go here http://www.nhlbisupport.com/bmi/?htf=6&hti=1&wt=192&bmi=24.4 and enter my body information, 6′0” and 191lbs, it tells me  my BMI is 25.9, overweight. Similarly, many athletes are considered overweight by the BMI calculation. It is not taking into account physiological differences between different people. Some people have more bone mass than others and fit individuals have higher levels of muscle tissue which weigh more than fat.  Back to me, I know for a fact from skeletal muscle analysis that I fluctuate between 11 and 13 percent body fat as percent total of body weight and have a 32 inch waist. Thus, I am not overweight, I am merely fit. The same would hold true for others using this system of analysis. It is a system whose basic logic is flawed. A obese person will always have a high BMI, but a high BMI does not mean an individual is overweight. Like me, they could be fit and very healthy.

It gets scary when premiums for health insurance vary according to your BMI and it makes the United States look rather outdated using a number as nonsensical as BMI. NPR reported on a grocery chain that had different surcharges for health insurance if they had high BMIs. A much more accurate way to assess obesity is by measuring waist circumference and the CDC has it listed on their page as an alternative way to assess your health risks related to obesity. More complex methods are even more accurate but are also more expensive.  These include body composition analyzers and hydrostatic weighing.

Many, including myself, believe that it is because these alternative methods cost more and involve more time and effort that they are not more prevalent in their use and that this is why BMI continues to be the most widely used measure of body health. Just do yourself a favor and if you really want to assess the status of your body either simply measure your waist circumference or buy a cheap pair of calipers or if you want to spend a little more and get a body composition analyzer. Both would give you far more accurate information than BMI can.