Purpose: A few studies investigated renal size using computed tomography (CT) in middle-aged and elderly patients while taking renal function into account. Thus, we aimed to assess kidney dimensions and their predictors in patients without known renal disease by multi-detector computed tomography (MDCT). Methods: This is a 6-month-long prospective observational study. Estimated glomerular filtration rate (eGFR) was calculated with CockcroftGault (CG) and modification of diet in renal disease (MDRD) formulas. Pole-to-pole kidney length (R1) was determined from coronal and sagittal oblique images. Kidney length (R2), width (R3), and parenchymal thickness were measured in axial slices. Results: The data of the 930 patients were included in this study. CG-eGFR was more closely correlated with R1, R2, and R3 than MDRD-eGFR. CG-eGFR, female gender, and presence of diabetes mellitus were independent predictors of R1 size in logistic regression analyses. Conclusion: Kidney sizes diminish with advancing age. CG-eGFR shows a better correlation with kidney dimensions compared with MDRD. Overall, age was a stronger determinant of eGFR than renal dimensions.