From Ed & Psych Associates:
Body Mass Index Calculator is an easily calculated index of body fat for adults that uses only height and weight. The formula used in this Calculator is (Weight in Pounds / Height in Inches squared) x 703. The definitional formula is Weight in Kilograms / Height in Meters squared. Because the BMI uses only height and weight, it does not take into account the distribution of muscle and bone mass. Thus, it may be incorrect for athletes or the elderly who generally exhibit unusual distributions of muscle and bone mass. The BMI should be considered in the context of other variables and used with caution, following consultation with a physician.