This article outlines the use of a Lean design process, enabled by simulation modeling, to determine the appropriate size of an emergency department based on current patient volumes and projected patient volumes in 12 years. In its most recent year, the emergency department hosted 67,000 patient visits within 34 exam rooms. Projections estimate that almost 74,000 annual patient visits within 56 exam rooms will be needed in this emergency department in 12 years. This study began with a process map of the patient flow within each acuity level. Thereafter, a simulation model was built to mimic the patient flow in the design of the new emergency department. Patient wait times were the key metric to assess the efficacy of the facility design. The results of this study revealed that the planned facility size was bigger than necessary and rooms could be eliminated from the plan and design, thus providing savings in construction.