It was shown that mercury(II) in aqueous solutions can be determined by standardless stripping voltammetry based on the determination of the coulometric constant of the electrochemical cell. This approach considerably reduces the analysis time in comparison to the known analogs. The coulometric constants were found for electrochemical cells with a gold and a gold-graphite film electrodes in solutions of different volumes. The optimum time of preelectrolysis was determined for measurements using the proposed method.