Relative stabilities of cucurbit[n]uril homologues (CB[n]; n = 4-7) and decamethylcticurbit[n]uril homologues (DCB[n]; n = 4-6) have been inspected with ab initio and density functional theory (DFT) calculations. Results show that CB [6] and CB [7] are more stable among CB [n] homologues and DCB [5] among DCB [n] homologues, which confirms the previous experiments. This is also supported in consideration of the puckering angles of the respective dimeric building units. Our investigation on the template effect in the formation of CB and DCB macrocyclic structure suggests that H3O+ is important in tethering, the building units being added to the macrocyclic structure in the course of the reaction. The presence of acid tends to favor the structures of smaller-size homologues. Thus, in consideration of the acid contents in the experiments, our calculations explain why CB [6] and DCB [5] are the major products. Moreover, it explains why considerable amount of CB[7] could be obtained in the recent report in which the amount of CB[7] was one-third of the amount of CB[6]. In a similar fashion, DCB[6] is predicted to exist as a coproduct with DCB[5].