BMI does not account for muscle.

As a measurement of an individual's percent of body fat using only height and weight and indicators, it would seem that the only way to achieve an accurate result is to somehow calculate muscle mass as well.  Oddly, the measurement is fairly accurate for the majority of the population as proven in research comparing the BMI measurement method with other more complicated methods such as the hydrostatic body fat test and dual energy x-ray absorptiometry.

When Is the Body Mass Index Inaccurate?

Experts agree that the body mass index isn't perfect, but it's a valuable tool in that it's easy to administer and understand for most people and can be a clear starting point to indicate that a more thorough health assessment is needed.  The problem with the test is that it does not take into consideration age, gender, or muscle mass so a small portion of the population will not get accurate results such as a heavily muscled athlete or an elderly person that has lost a lot of muscle.  According to WebMD this very scenario happened to Michael Jordan, who in his prime had a body mass index of27-29, which by the body mass index standard weight status categories indicated that he was overweight.

Waist Circumference May Be More Accurate

For athletes and those that are heavily muscled a more accurate indicator of body fat percentage may be the waist circumference method of measurement.  Many experts believe that this is an important method of screening for possible health risks associated with obesity since many conditions such as heart disease and type 2 diabetes increase as the waist size increases. According to the National Heart and Lung Institute the risk goes up with a waist measurement larger than 35" for women and 40" for men.