A theoretical model is given for calculation of parasitic currents (bypass currents) for a stack of bipolar cells used in molten salt electrolysis. A simplified bypass current calculation was confirmed by the numerical solution of the Laplace equation for Galvani potentials in the interelectrode space and in the free space of the cell stack for the aluminium cell of the ’Alcoa Smelting Process’ (AlCl3 electrolysis) with 11 bipolar electrodes. Due to the bypass currents the current efficiency of the electro-chemical process was lowered to 82%-94%, depending on the height of the stack, i.e. the thickness of the bipolar electrodes. These values were in a good agreement with values obtained on the basis of the simplified approach. The numerical solution of the Laplace equation allowed a detailed description of the influence of the bypass current flow on the current distribution at the edges of the bipolar electrodes.