A new criterion and potentiostatic method is demonstrated to determine the electrochemically stable potential window (ESPW) of supercapacitors, enabling the calculation of the optimal electrode mass ratio for two different electrode-electrolyte pairings. The criterion uses the decay of electrode current in a three-electrode test cell during potentiostatic ageing at progressively higher voltages to determine the ESPW. To perform these measurements a quasi-reference electrode (QRE) was incorporated within a CR2032 coin cell with a small and simple modification. This enabled three-electrode measurements to be performed inside the coin cell using organic electrolytes whilst outside a glovebox. The voltage-optimised mass ratio (m+/m−) for activated carbon electrodes in 1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (ACN) and propylene carbonate (PC) electrolytes was shown to be 2.38 and 0.61 respectively. The validity of the technique was verified by full-cell potentiostatic ageing experiments on mass-balanced and mass-symmetric supercapacitors. For both electrolytes the maximum operating voltage, energy storage capacity and cycle lifetime were improved significantly with mass balancing. The maximum voltage increased from 2.0 V to 2.9 V for ACN and from 2.3 V to 2.7 V for PC, whilst the energy density increased by ∼90% for ACN and by ∼40% for PC.