Operating room (OR) efficiency has an impact on surgeon productivity and patient experience. Accuracy of case duration estimation is important to optimize OR efficiency. The purpose of this study was to identify factors associated with inaccurate case time estimates in outpatient hand surgery. A better understanding of these findings may help to improve OR efficiency and scheduling.All outpatient hand surgical cases from 2018 to 2019 were reviewed. Poorly-estimated cases (i.e., poor scheduling accuracy) were defined as those cases where the actual operative time differed from the predicted time by50% (either quicker by50% or slower by50% than the predicted time). The percentages of poorly-estimated cases were analyzed, categorized, and compared by surgeon, procedure type, and scheduled case length.A total of 6,620 cases were identified. Of 1,107 (16.7%) cases with poorly estimated case durations, 75.2% were underestimated. There was no difference in the likelihood of poor estimation related to start time. Well-estimated cases tended to have longer scheduled case duration, but shorter realized case duration and surgical time. Our systems analysis identified specific surgeons and procedures as predictable outliers. Cases scheduled for 15-30 minutes frequently were inaccurate, whereas cases scheduled for 30-45 and 106-120 minutes had accurate estimates.The accuracy of case time estimations in a standard outpatient hand surgery practice is highly variable. Nearly one-fifth of outpatient hand surgery case durations are poorly estimated, and inaccurate case time estimation can be predicted based on surgeon, procedure type, and case time.Maximizing OR efficiency should be a priority for surgeons and hospital systems. With multiple surgeries done per day, the efficiency of the OR has an impact on surgeon productivity and patient experience.