This paper presents a probabilistic based software tool for estimation of digital systems’ testability. The tool allows fast computation to estimate testability in linear time complexity regarding the number of components and interconnects of the digital system. The designed tool is based on utilization of controllability and observability measurement for the estimation of overall system’s testability. The algorithmic procedures are tested, verified and demonstrated through a large number of simulated digital systems. As examples some of the simulated results are embedded in this paper.