We introduce an open-source software for fire risk assessment named Aamks. This article focuses on a component of Aamks—an evacuation simulator named a-evac. A-evac models evacuation of humans in the fire environment produced by a zone fire model simulator. In the article, we discuss the probabilistic evacuation approach, automatic planning of exit routes, the interactions amongst the moving evacuees and the impact of smoke on humans. The results consist of risk values based on fractional effective dose and are presented in the form of various probability distributions and evacuation animations. The intended scope of Aamks is buildings, e.g., offices, malls, factories rather than stadiums or streets. We present the need for such software based on the current state of research and existing engineering tools in the probabilistic risk assessment domain. Then we describe a-evac and its details: geometry, path-finding, local movement, interaction with fire, and visualization. Given the above scope, the article contributes to the domain of probabilistic risk assessment by proposing: (a) stochastic approach to evacuation, (b) velocity-based model for evacuation, (c) evacuation software that interacts with fire conditions and zone fire models.