Purpose The American College of Surgeons (ACS) Surgical Risk Calculator (SRC) was developed with numerically robust surgical interactions. Previous analysis suggest the ACS SRC underestimates post-operative risk. This study analyses the external validity of the ACS calculator’s assessment of risk. Methodology Patients underwent radical cystectomy and ileal conduit formation at Fiona Stanley Hospital (FSH) between January 2016 and March 2024 for urothelial malignancy. Average and patient-estimated risk from the ACS NSQIP database, and observed outcomes of the 141-patient cohort were tabled. Spearman’s test was used to analyse the correlation groups (p-value <0.05 = significance). A p-value adjustment was performed using Hommel’s method. Average calculated serious complications of the FSH cohort were compared against Clavien-Dindo Classification scores using Chi-Squared test of proportions. Results Of the 141 participants, 113 (80%) were males and 28 (20%) females. There were no deaths within 30 days of the operation. The null hypothesis, variables are not correlated, was excluded (true Rho = 0). Accepting the alternate hypothesis, correlation exists between the FSH cohort calculated risk and the observed outcome (Z = 3.076, P-value 0.002). Correlation also exists between ACS cohort average risk and observed outcome (Z = 3.076, P-value 0.002) as well as FSH cohort calculated risk and ACS average risk (Z = 3.464, P-value 0.0005). Using the Clavien-Dindo Classification as an alternate method of identifying ‘serious complication’, the observed outcome is 9.9%. Conclusion Prior literature assessing the validity of the ACS SRC is varied but favours underestimation of 30-day post operation surgical risk. We have shown it can be an accurate tool for guiding clinicians and patients on post-operative risks after radical cystectomy. The criteria for serious complications in the ACS SRC are broad and non-specific and may exaggerate serious complication risk.