A convenient and fast method to determine the composition of LiNbO3 single crystals consisting of a measurement of the position of the fundamental absorption edge in the near UV region is refined and extended to the stoichiometric range. Using unsophisticated apparatus and simple reflection correction, the proposed two-parameter calibration equation provides an absolute accuracy of 0.1 mol % with the relative accuracy reaching an unusual value of better than 0.01 mol % for near stoichiometric compositions.