BackgroundThe length of a patient's stay (LOS) in a hospital is one metric used to compare the quality of care, as a longer LOS may flag higher complication rates or less efficient processes. A meaningful comparison of LOS can only occur if the expected average length of stay (ALOS) is defined first. This study aimed to define the expected ALOS of primary and conversion bariatric surgery in Australia and to quantify the effect of patient, procedure, system, and surgeon factors on ALOS.MethodsThis was a retrospective observational study of prospectively maintained data from the Bariatric Surgery Registry of 63 604 bariatric procedures performed in Australia. The primary outcome measure was the expected ALOS for primary and conversion bariatric procedures. The secondary outcome measures quantified the change in ALOS for bariatric surgery resulting from patient, procedure, hospital, and surgeon factors.ResultsUncomplicated primary bariatric surgery had an ALOS (SD) of 2.30 (1.31) days, whereas conversion procedures had an ALOS (SD) of 2.71 (2.75) days yielding a mean difference (SEM) in ALOS of 0.41 (0.05) days, P < 0.001. The occurrence of any defined adverse event extended the ALOS of primary and conversion procedures by 1.14 days (CI 95% 1.04–1.25), P < 0.001 and 2.33 days (CI 95% 1.54–3.11), P < 0.001, respectively. Older age, diabetes, rural home address, surgeon operating volume and hospital case volume increased the ALOS following bariatric surgery.ConclusionsOur findings have defined Australia's expected ALOS following bariatric surgery. Increased patient age, diabetes, rural living, procedural complications and surgeon and hospital case volume exerted a small but significant increase in ALOS.Study typeRetrospective observational study of prospectively collected data.