We found that field-effect mobility, which had been widely used in the evaluation of the mobility of an amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistor (TFT) failed to describe the effect of mobility on propagation delay ( ${t}_{rm PD}$ ) in an a-IGZO TFT-based circuit, and also proposed an extraction technique for the ${t}_{rm PD}$ -correlated mobility ( $mu _{{{t_{rm PD}}}}$ ) considering both the subgap density-of-states and the voltage-dependent charge density. It is verified that the proposed $mu _{{{t_{rm PD}}}}$ is the best correlated with the measured ${t}_{rm PD}$ in IGZO TFT-based inverters other than various mobilities in the literature. Our results have revealed that it is possible to predict ${t}_{rm PD}$ only with the measured current–voltage characteristic of the a-IGZO TFT without measuring ${t}_{rm PD}$ in IGZO-based circuits.