Hospitals, particularly emergency departments, are vital to a community’s resilience and earthquake recovery by providing emergent and ongoing critical care. Building standards and codes have higher structural requirements for hospital structures to help ensure continued functionality; nevertheless, hospitals are often damaged, evacuated or closed following design-level earthquakes. While post-earthquake hospital functionality has been the focus of recent research, it remains challenging to predict hospital functionality in a relevant and meaningful manner. Existing methods to quantify functionality are based on patient wait time, bed count, or available services. This work focuses on improving existing hospital functionality frameworks to predict various levels of hospital functionality using a combination of fault and event trees to provide a probabilistic prediction of different levels of functionality based on available services and bed counts for hospital emergency departments and whole hospitals. Fault and event trees were created using published data on hospital damage, evacuations, and operations following earthquakes and combined with international guidelines on hospital requirements and local hospital emergency procedures. The fault and event trees relate physical damage, staff shortages, and equipment failures to various hospital and emergency room functionality levels. Event trees provide a sequence of events (availability of critical staff, structural damage, and non-structural damage) leading to various levels of hospital functionality (full, partial, or not functional). Each event has an associated probability of occurring determined by a developed fault tree specific to that event failure. The methodology was validated using an additional hospital dataset to hindcast the functionality of the hospital facility. The hindcasted results match the published functionality data, indicating that this method provides a way to predict levels of functionality, which can indicate what types of services and space will be available for treatment following an earthquake.