Background Formulae of estimated glomerular filtration rate (eGFR) based on serum creatinine (Scr) are routinely used in oncology patients, however, they are inaccurate in some populations. Our aim was to identify population characteristics where eGFR formulae performed poorly and thereby build a nomogram to predict the reliability of estimates. Methods Measured GFR (mGFR) using isotope from 444 oncology patients were compared with eGFR from four formulae (Cockcroft-Gault, Modification of Diet in Renal Disease (MDRD), Chronic Kidney Disease Epidemiology (CKD-EPI) Collaboration and Wright). Multivariate logistic regression was applied to identify characteristics associated with inaccurate eGFR and construct a predictive nomogram. Results The Cockcroft–Gault formula exhibited estimates with lowest bias and highest precision. Nonetheless, it was still unreliable in a relevant proportion of patients. The percentage of patients within 30%, 20%, and 10% of the accurate percentage error (APE) was seen in only 62.8%, 47.7% and 24.8% of patients respectively. Inaccuracy was found in overweight patients or in patients with BUN/Scr ratio greater than 20 or with eGFR greater than 120 ml/min. A novel nomogram was constructed to help oncologists to predict the risk of inaccuracy of eGFR. The calibration curve showed good agreement. Conclusions Our results suggest that all eGFR formulae tend to overestimate the eGFR in oncology patients. Our nomogram may assist oncologists in decision-making when mGFR is needed.