With the increasing complexity in software and electronics in safety-critical systems new challenges to lower the costs and decrease time-to-market, while preserving high assurance have emerged. During the safety assessment process, the goal is to minimize the risk and particular, the impact of probable faults on system level safety. Every potential fault must be identified and analysed in order to determine which faults that are most important to focus on. In this paper, we extend our earlier work on formal qualitative analysis with a quantitative analysis of fault tolerance. Our analysis is based on design models of the system under construction. It further builds on formal models of faults that have been extended for estimated occurence probability allowing to analyse the system-level failure probability. This is done with the help of the probabilistic model checker PRISM. The extension provides an improvement in the costly process of certification in which all forseen faults have to be evaluated with respect to their impact on safety and reliability. We demonstrate our approach using an application from the avionic industry: an Altitude Meter System.