Context: To ensure equitable access to prehospital care, as recommended by the Rural and Frontier Emergency Medical Services (EMS) Agenda for the Future, policymakers will need a uniform measure of EMS infrastructure. Purpose and Methods: This paper proposes a county-level indicator of EMS resource availability that takes into consideration existing EMS resources (ambulances), population health and demographics, and geographic factors. The indicator, the Expected annual emergency miles per AMBulance (EXAMB), provides a basis for comparing ambulance availability across counties within states. A method for calculating the EXAMB indicator is demonstrated using data from 5 states. Findings: The EXAMB indicator was negatively correlated with ambulance availability per 100,000 population in 4 of the 5 states in the study. The indicator was positively correlated with rurality in 3 states. In Mississippi, South Carolina, and Wyoming, whole-county health professional shortage areas had median EXAMB values 45%-81% higher than those of the non-health professional shortage areas counties. Conclusions: Future research should explore the relationship of the EXAMB to EMS outcomes, with the ultimate goal of developing a nationally recognized indicator of "adequate" EMS resource availability.