Comparing the performance of thermophotovoltaic (TPV) devices is challenging due to a lack of standard operation conditions. Here, we propose a universal figure of merit (FOM) that can be used to evaluate the performance of TPV devices that operate in the far-field regime relative to their thermodynamic bounds. The introduced FOM alleviates temperature dependence and accounts for the fundamental trade-off between power density and efficiency. Based on this FOM, we present a classification of TPV performances reported in recent experiments.