The most common workload surrogates used by Emergency Medical Services (EMS)--overall call volume (CV) and unit hour utilization (UHU)--have not been validated. This study investigates the relationship between EMS operational metrics and perceived workload, measured through the NASA task load index (TLX) survey. We paired crew member survey responses with the operational metrics based on dispatch data. Linear regression was used to evaluate the influence of each metric on perceived workload. Logistic regression was used to identify values of workload indicating a fair day's work. We found that the overall CV (p < 0.001, =0.32) and UHU (p < 0.001, =0.22) explained less variability in perceived workload than a priority (P)-stratified CV (p < 0.001, =0.41). However, we found no clear fair day's workload threshold. We conclude that a priority-stratified CV is a better workload surrogate than a CV and UHU. © 2024 Elsevier Ltd