KT/V, the normalized whole-body urea clearance, is becoming accepted as a standard measurement for monitoring the adequacy of hemodialysis. Current methods for its calculation are limited by inaccurate knowledge of dialyzer urea clearance (K) and urea volume of distribution (V). We report a simple formula for calculation of KT/V from pre-, post- and mid-dialysis blood urea nitrogen. Evaluation of 37 hemodialysis treatments by measurement of total dialysate urea revealed that (1) urea kinetic modeling consistently overestimated V; (2) conventional arteriovenous sampling overestimated K by a mean of 23%, and (3) KT/V derived from the simple formula was compared to values generated by four other methods and found to more precisely estimate true KT/V.