Availability of a clustered system is determined by identifying all possible states of a cluster, mapping the connections between the states are mapped and determining the probability of the cluster being in each of the states is determined. Availability of the cluster is the sum of the probabilities of being in those states in which the cluster is at least partially available. Parameters affecting the availability of the cluster include the rates at which a cluster transitions from state to state, including the reciprocal of Mean Time To Fail, Mean Time to Fail-Over, Mean Time To Fail-Back and Mean Time To Restore. Availability of statefull and stateless clusters may be determined. Availability of clusters including any number of nodes (e.g., a Internet service) can be determined. In one embodiment of the invention, a target availability is determined and parameters are varied to determine the most efficient approach to reach the target availability.