The authors present a formula to evaluate the performance of unbuffered multistage interconnection networks (MINs), with emphasis on MINs composed of 2 × 2 switching elements. In this formula, the normalized throughput of an unbuffered MIN is expressed in terms of the input rate and the number of states. The new formula is proved by mathematical induction. The major advantage of this formula is that the asymptotic behavior of an unbuffered MIN can be easily determined. It is shown that the formula results in the same asymptotic performance as that obtained by C. P. Kruskal and M. Snir (1983). In addition to analyzing the asymptotic behavior, the formula can be applied to evaluate the exact performance of unbuffered MINs. The formula is generalized for unbuffered MINs in the presence of nonidentical input rates.