Abstract: The method proposed in the paper uses the finite state machine (FSM) to abstract interface system design specifications and calculate the probabilities of FSM states transition using probabilistic regular grammars. We present a usability algorithm considering states transition probabilities and users' proficiency. Finally we take a mobile phone interface as a case for study to validate the method. Since this method can be employed in early phases of interface system life cycles, it can be used to compare different candidate designs, and furthermore, to decrease development risks.