Estimating Surgical Duration for Surgeries: Towards an Efficient Schedule of Operating Theatre Poor operation theatre (OT) scheduling is a common cause of case cancellation on the day of surgery. We analysed the accuracy of anaesthetists and surgeons in estimating the surgical time to complete an operation and investigated whether the estimated times was correlated with the actual surgical time. It was a prospective observational study with 369 elective cases from various surgical subspecialties which were performed within three months in a tertiary university hospital. Anaesthetists and surgeons were required to estimate the surgical time of the elective cases before the day of surgery. Their estimations were compared with the recorded actual surgical time. The primary outcome was the median time difference from the estimated surgical time. Anaesthetists accurately estimated surgical times of 82 cases (22%) and surgeons 59 cases (16%). Overall, anaesthetists overestimated the surgical time by 11 minutes of actual surgical times [interquartile range (IQR) -15 to 40 minutes]. Surgeons underestimated their surgical time by 15 minutes [IQR -60 to 20 minutes]. The median estimated surgical time difference between anaesthetists and surgeons was significantly different (p<0.0001). Among the surgical subspecialties, the longest overestimation by anaesthetists was up to 48 minutes in colorectal surgeries. The most underestimated median time difference by surgeons was 65 minutes, seen in upper gastrointestinal surgeries. A significant strong positive correlation existed between overall anaesthetists' estimated and actual surgical times (rs = 0.735, p<0.0001). At the same time, there was a weak correlation between surgeons’ overall estimation time upon booking and actual surgical times (rs = 0.492, p<0.0001). In conclusion, both the anaesthetists and surgeons were significantly inaccurate in estimating the duration of surgical procedures. Although the anaesthetists tended to overestimate it while surgeons underestimated it, their estimations correlated strongly with the actual surgical times, whereas surgeons’ estimations were weakly correlated