Redox flow batteries (RFB) are a promising grid-scale electricity storage technology due to their decoupling of capacity (dissolved amount of redox active species (RAS) in the electrolyte) and power (cell potential of the RAS). However, as there are still no standard setups and operating parameters for RFBs, the comparability of measurement results is limited due to the large variety of different RFB setups. To tackle this limitation, a mass transfer model and an experimental procedure are presented here that are based on productive (nV) and consumptive molar fluxes (nI), enabling a performance evaluation (K1 and K1critical) and comparability between different cell setups by introducing a RFB cell constant (ζ and K2).