Protection against fire is one of the pillars of maritime safety. In particular for passenger ships, experience has shown that a fire onboard can incur catastrophic consequences. Although the maritime fire safety legislation requires compliance with prescriptive regulations of fire prevention, protection and extinction, the concept of “safety equivalence” has enabled substantial innovations in the design of some passenger ships. The assessment framework could be strengthened by introducing a probabilistic formulation. The present study is an attempt for a step forward, aiming to develop the elements and the structure of a probabilistic attained fire safety index. The probability of fire ignition, the reliability of the installed suppression systems and the expected loss due to fire growth, are key elements contributing to the novel assessment which is sensitive to design detail, such as the space layout and the interior design materials used. The distribution of fatalities has been selected as the most appropriate “loss” function. As an application, two adjacent fire zones accommodating passenger spaces will be examined. The loss function distribution is derived from a batch of fire and evacuation simulations. The method is extendable to address the entire ship.