In this paper we introduce a notion of fault-tolerance distance between labeled transition systems. Intuitively, this notion of distance measures the degree of fault-tolerance exhibited by a candidate system. In practice, there are different kinds of fault-tolerance, here we restrict ourselves to the analysis of masking fault-tolerance because it is often a highly desirable goal for critical systems. Roughly speaking, a system is masking fault-tolerant when it is able to completely mask the faults, not allowing these faults to have any observable consequences for the users. We capture masking fault-tolerance via a simulation relation, which is accompanied by a corresponding game characterization. We enrich the resulting games with quantitative objectives to define the notion of masking fault-tolerance distance. Furthermore, we investigate the basic properties of this notion of masking distance, and we prove that it is a directed semimetric. We have implemented our approach in a prototype tool that automatically compute the masking distance between a nominal system and a fault-tolerant version of it. We have used this tool to measure the masking tolerance of multiple instances of several case studies.