Objective: Evaluate the useful length and the diameter of the cochlear lumen (CL) using routine imaging before cochlear implantation to study inter-individual variability and its impact on the insertion depth of the electrode carrier (EC). Study design: Prospective cross-sectional study. Setting: Tertiary referral center. Patients: Thirty-one preoperative and postimplantation temporal bone CT scans were analyzed by two investigators. Intervention: Images were analyzed via orthogonal multiplanar reconstruction (Osirix) to measure the lengths of the entire CL and the basal turn. By means of curvilinear reconstruction, the CL was unfolded and the diameters of the CL and of the EC were measured every 2 mm from the round window (RW) to the apex. Results: Very high-inter individual variability was found for the length of the basal turn (RSD > 1000%), the entire CL length (RSD > 800%), and the CL diameter at the RW (RSD > 600%). CL diameter was not correlated to the CL length. The inserted EC/total visible CL length ratio was 1.0 ± 0.12. Reliability of the measures was acceptable for the CL length and the diameter at 16 mm from the RW (Crohnbach's alpha > 0.7, n = 31). Conclusion: CL length and diameter can be directly measured in a reliable manner by commercially available tools. These parameters potentially influence the EC insertion and should be assessed before cochlear implant surgery.