New electrochemical reactors with high specific electrode surface area and low investment and operation cost are needed for the industrial application of electrochemistry. Due to its high productivity and low cost, the bipolar electrochemical reactor is a very promising candidate for industrial application. The main disadvantage of the bipolar electrochemical reactor is the presence of parasitic electrical currents, or current bypass at the lower and upper parts of the electrode stack. For the scale-up, a relation between the current bypass (psi) and two dimensionless numbers (Gb and Bn) has been derived. psi = Gb(Bn + 1). The bipolar number Bn depends on the electrochemical system used and on the process parameters (sigma, i0) in contrast to the geometric number Gb, which depends only on the geometry of the bipolar reactor. Measured current bypass in a bipolar electrode stack demonstrates the validity of the scale-up relation for psi less-than-or-equal-to 0.68.