Introduction: The aim of this study was to compare the tooth length measured on already existing cone beam computed tomography (CBCT) scans, the reading of electronic apex locater in vivo and the actual length of the same tooth after extraction. Methods: A total of 178 canals were measured three times in this study. The aim of this study was to compare the tooth length measured on already existing CBCT scans, then clinically with the aid of two different electronic apex locators. After that, teeth were extracted, and the actual length of the canals was measured and considered as the gold standard. The means and standard deviations of electronic apex locators' lengths, CBCT and actual lengths in the mesiobuccal, distobuccal and palatal roots were statistically analysed using the one-way ANOVA test (P ≤ 0.05). Results: No statistically significant difference was found between the CBCT measurements, the apex locator readings and the actual tooth length. Conclusion: A pre-existing CBCT scan can reliably be used for tooth length determination. Keywords: actual tooth length, CBCT, electronic apex locator, Rootor, Root ZX mini