The Directional Cycle decomposition Algorithm (DCDA) is one method to achieve full restoration and tolerate as many faults as possible in wavelength division multiplexing (WDM) networks without the capability of wavelength conversion. This paper formulates the restorability computing in WDM network based on DCDA. Two methods are proposed to compute the restorability. One computes the restorability exactly using the information from the original network and its corresponding G+. Another computes approximately using the information only from the corresponding G+. The results indicate the restorability is decreasing very fast with more and more simultaneous faults. Moreover, the average error in the approximate method is limited from 8.37% on ArpaNet to 2.94% on Icosahedron.