Background: We assessed the accuracy of four estimated glomerular filtration rate (eGFR) methods: MDRD, Cockcroft–Gault, CKD-EPI, and Wright. Method: The four methods were compared to measure GFR (mGFR) in patients with urothelial urinary tract cancer (T2-T4bNxMx) receiving platinum-based chemotherapy at Rigshospitalet, Copenhagen, from January 2019 to December 2021. Using standardized assays, creatinine values were measured, and mGFR was determined using Technetium-99 m diethylenetriaminepentaacetic acid (Tc-99 m-DTPA) or Cr-51-ethylenediaminetetraacetic acid (Cr-51-EDTA) plasma clearance. Patients (n = 146) with both mGFR and corresponding creatinine values available were included (n = 345 measurements). Results: The CKD-EPI method consistently demonstrated superior accuracy, with the lowest Total Deviation Index of 21.8% at baseline and 22.9% for all measurements compared to Wright (23.4% /24.1%), MDRD (26.2%/25.5%), and Cockcroft–Gault (25.x%/25.1%). Bland Altman Limits of agreement (LOA) ranged from − 32 ml/min (Cockcroft–Gault) to + 33 ml/min (MDRD), with CKD-EPI showing the narrowest LOA (− 27 ml/min to + 24 ml/min and lowest bias (0.3 ml/min). Establishing an eGFR threshold at 85 ml/min—considering both the lower limit of agreement (LOA) and the minimum cisplatin limit at 60 ml/min—allows for the safe omission of mGFR in 30% of patients in this cohort. Conclusion: CKD-EPI equation emerged as the most suitable for estimating kidney function in this patient group although not meeting benchmark criteria. We recommend its use for initial assessment and ongoing monitoring, and suggest mGFR for patients with a CKD-EPI estimated GFR below 85 ml/min. This approach could reduce costs and decrease laboratory time for 30% of our UC patients.