BMI was developed in the early 1800's (a very loud alarm should have sounded just knowing that piece of information) by a Belgian mathematician. Somehow this genius calculated that weight (in pounds) divided by height (in inches) squared, multiplied by 703 determined an individual's body fat.
The Centers for Disease Control, for some unknown reason, sees this formula as a viable solution for calculating body fat. There's just one issue. The matter of determining the BMI by using only a person's body weight and not their composition. Meaning what exactly? Well, the cartoon above is a perfect representation of this. Both characters above are the same exact height and weight. The index states that both of these individuals have the same BMI, meaning the bodybuilder with 6% body fat has the same exact BMI as the man who is more than 30% body fat.
Clearly what I've just stated is not only wrong, it's VERY wrong, and should be aborted as a means to determine a person's body fat. A person is much more than just their height and weight. Everyone has muscle, bone, and water as well as fat and these must be taken into account to get a proper idea of their fat percentage.
Want to know what your real body fat percentage is? Have a fitness/health professional weigh, measure, and use skin fold calipers on you or see about getting access to a bioelectrical impedance device. Many gyms or personal trainers have access to one or both of these devices. Whatever you do, don't get upset over your BMI. Odds are it's wrong anyway.