Introduction The ultimate success of fixed prosthesis is to pay meticulous attention to the detailed production of the missing tooth structure. Such an accurate replication is technically demanding. Hence, the aim of the study was to evaluate the accuracy of marginal fit of castings with varying sprue diameter and using ring and ringless techniques. Materials and methods Forty standardized wax copings were fabricated on a stainless steel die and divided into two groups. The first group was cast using the metal ring investment technique; the second group was cast using the ringless investment technique. Both the groups were further divided into two groups, each based on the diameter of the sprue (groups I and III: 4.5 mm and groups II and IV: 2.5 mm). The vertical marginal gap was measured at three sites per specimen, using a digital optical microscope at ×100 magnification. The results were analyzed using two-way analysis of variance and independent t-test to determine statistical significance. Results The vertical marginal gaps of castings fabricated using the ringless technique (0.145 ± 0.072 mm) were significantly less (p < 0.001) than those castings fabricated using the conventional metal ring technique (0.264 ± 0.100 mm); however, the vertical marginal gaps of the 2.5 mm sprue diameter (0.199 ± 0.092 mm) and 4.5 mm sprue diameter (0.211 ± 0.118 mm) castings were not statistically significant (p > 0.05). There was also statistically significant interaction (p < 0.001) between group IIA (0.120 ± 0.038 mm) and group IA (0.302 ± 0.098 mm), which was analyzed using the independent t-test. Conclusion It can be concluded that the ringless casting technique in comparison to metal ring technique produces metal castings of better marginal adaptation. Furthermore, the sprue diameter had significant difference wherein the 4.5 mm diameter produced the least vertical discrepancies in the ringless casting technique and 2.5 mm produced lesser vertical discrepancies in the metal ring. Clinical significance One of the key factors that decides the success of a metal-ceramic restoration is the marginal fit. The accurate fit of the restoration greatly minimizes the plaque accumulation, offers good mechanical properties, and reduces micro-leakage and development of secondary caries. The techniques of casting procedure and sprue diameter can have an influence on the outcome. Hence, the current research was undertaken to study the influence of these factors on marginal fit. How to cite this article Dutta T, George VT, Balakrishnan D, Narayan AI. An in vitro Comparative Study to evaluate the Marginal Fit of Castings using Ring and Ringless Casting Techniques with Varying Sprue Diameter. World J Dent 2017;8(2):109-113.