Ship structures must be designed with adequate safety and reliability, and their designs must be acceptable from an environmental and economical point of view. Target reliabilities have to be met in the design rules in order to ensure that certain safety levels are reached in design according to the rules. There are several steps involved in a process of determining target reliabilities in agreement with a risk based approach as recommended by Formal Safety Assessment, IMO (1997, 2001). The study concentrates on the second step of FSA, i.e. calculation of the reliability level inherent in existing rules representing past practice when state-of-the-art models for environment, loads, response and capacity are adopted. Buckling of a ship deck in the extreme sagging conditions is considered. The suggested procedure is illustrated by three examples. The reliability calculations are carried out for the ‘as build’ stiffened deck plate thickness. Uncertainties involved in the suggested analysis procedure and their consequences on implicit reliability are presented. Implementation of the results in the risk analysis is discussed.