Objectives: Length of stay (LOS) is an important emergency department (ED) performance measure. The objective of this study was to predict the 10th, 50th and 90th percentile of ED LOS using demographic, clinical and temporal characteristics in order to better inform patients and ED staff. Methods: A retrospective cohort study was conducted on one year ED visit data from an academic ED (N=50,824). We estimated the 10th, 50th and 90th percentile for three different phases of ED care: waiting time, treatment time and boarding time. We used multivariate quantile regression to model the three phases of ED care separately as a function of patients' arrival day and time, age, gender, mode of arrival, insurance status, acuity level and chief complaint. Results: The median waiting time was 14 minutes, 191 minutes for median treatment time and 154 minutes for median boarding time. Patients at the 90% waited 7 times longer (98 minutes), took 2.5 times longer to be treated (487 minutes) and boarded 7 times longer (1,122 minutes) compared to patients at the median. Patients' chief complaint and acuity level were the most important predictors of the three phases of LOS. The adjusted median treatment times for patients with a cardiovascular symptom (202 to 328 minutes depending on acuity level) were longer than patients with any other complaint, regardless of acuity. Of all chief complaints, the longest median boarding times were experienced by patients with a skin problem (177 to 291 minutes depending on acuity level). Day and time of arrival were important predictors of wait time and boarding time as well. The adjusted boarding times were 30% to 100% longer on Mondays compared to Sundays. Conclusions: ED LOS varied significantly among patients in a predictable manner that is largely explained by information available at triage. Providing patients with an expected LOS at triage may result in increased patient satisfaction.