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Iridium oxide is one of the most common anode catalysts in commercial PEM electrolyzers for its high oxygen evolution reaction (OER) activity and stability. Unfortunately, benchmarking iridium oxide OER catalysts has proven difficult since IrO2 cannot undergo proton underpotential deposition, which makes it difficult to estimate the electrochemically active surface area (ECSA). In this work, we propose a method to calculate the electrochemically active surface area of iridium oxide in an operating PEM electrolyzer. A constant of 601 mC/cm2 was obtained using powder electrocatalysts by correlating the pseudocapacitive charge and electrochemically active surface area of the iridium oxide. This allowed us to determine the catalyst specific and mass activities in an operating PEM electrolyzer.