Electrochromic devices (ECDs) are extensively used in various applications. While multiple compositions of electrochromic, electrolyte, and ion storage layer materials have been investigated, design guidelines that account for the impact of thickness of these layers on the optical response of ECDs remains unexplored. The optical performance of ECDs have been analyzed using the transfer matrix method. Our calculations show the significance of optimizing thickness of different layers in ECDs, for obtaining high-performance devices. We have illustrated the same by applying these analysis techniques to experimentally fabricated devices reported in literature. We believe that this insight will aid in the design of ECDs.