This paper presents a metric to estimate the register file criticality in processor-based systems. Due to project constrains, it is mandatory to identify and prioritize the most critical registers to protect when a selective fault mitigation approach is needed. The metric is based on the combination of three different criteria, which are computed dynamically during run-time. The applicability and accuracy of the metric have been evaluated in a set of applications running in the miniMIPS and PicoBlaze microprocessors.