QoS routes need availability assurance. Regardless of the applied protection scheme, there are several heuristic algorithms that can approximate the availability of connections based on the availability parameters of network components along the connection path. Accurate approximation can be achieved with simple and fast calculation only for the most basic protection schemes, the more complex the protection scheme is the more complex and long running calculation is required to get the result. Though, the inaccuracy of the heuristic algorithms is negligible in the most cases of practical usage. In this paper we present an availability calculation method for p-cycles that exploits the special properties of the p-cycle protection scheme and provides accurate results without enumerating all the possible network or protection configuration states. The method evaluates the availability of the connection along the working path link-by-link incrementally, pre-calculating also those conditional availabilities of the connection-part that may be used in latter calculations. Theoretically, the complexity of the algorithm is still O(2n), however, in fact n does not get high, moreover, it can be kept moderate low if the size of the cycles is constrained.