The aim of this paper is to propose a decision support tool for contractors before the bidding stage to quantify the probability of delay in construction projects in Turkey by using the relative importance index (RII) method incorporated into fuzzy logic. For this purpose, 83 delay factors were identified, categorized into 9 major groups through a detailed literature review process as well as interviews with experts from the construction industry. The relative importance of these delay factors and groups was quantified by using the relative importance index method. The ranking of the factors and groups was demonstrated according to their level of effect on delay. A delay assessment model was proposed using the fuzzy set theory by taking into account the delay factors characterized in construction projects. The assessment model was developed using a commercial software product. The proposed methodology was tested in a real case study; and the probability of the delay of the project was evaluated by means of the assessment model after the required inputs were entered into the software. The factors and groups that contribute most to the probability of delays were discussed depending on the case study results. The assessment model results were found to be satisfactory. Finally, some recommendations were made in order to minimize and control delays in construction projects.