Quality of Service (QoS) analysis and prediction for Web service compositions is an important and challenging issue in distributed computing. In existing work, QoS for service compositions is either calculated based on constant QoS values or simulated based on probabilistic QoS distributions of component services. Simulation method is time consuming and can not be used in real-time applications for dynamic Web service compositions. In this paper, we propose a calculation method to estimate the QoS of a service composition, in which the probability distributions of the QoS of component services can be in any shape. Experimental results show that the proposed QoS calculation approach significantly improves the efficiency in probabilistic QoS estimation.