There is an ongoing effort to improve the energy storage capacity of graphene-based supercapacitors. These supercapacitors store energy in the electric field between the charge carriers in the graphene and the counter ions in a liquid electrolyte. To characterize this double layer, we use the Hall effect to determine the charge carrier density in graphene as a function of voltage. We disentangle the separate roles of double-layer capacitance (the electrostatic contribution) and quantum capacitance and compare the performance of different electrolytes. Our results highlight the advantages of Hall effect measurements for probing the electrostatics of graphene–electrolyte interfaces.