Complementary electrochromic devices (ECDs) have advantages in offering deeper switching over singer-layer ECDs. Therefore, it is important to gain an insight into the dominant factors that control the electrochromic performance of complementary ECDs. In this study, the design of complementary ECDs with two reversible redox electrodes operating in series is presented. By combining the electrochemical and optical properties of these two electrodes, design equations suitable for complementary ECDs are derived. As the overall extent of electrochromic reaction involved in an ECD is limited by the electrode with a lower charge capacity, the effect of charge capacity ratio on the optical attenuation range can be predicted. The required parameters in the design equations are evaluated from the fabricated cell. As an example, the tungsten oxide–Prussian blue system is discussed. The empirical prediction of the optical attenuation range is fitted well with the experimental results.