ABSTRACT Previous studies on the Cost of Quality (COQ) have focused on direct costs, employing prevention, appraisal, and failure costs (PAF). However, the advent of ISO 9001–2015] edition underscores the importance of the risk approach in quality management, necessitating the inclusion of project risks in COQ calculation. This paper introduces a prototype Fuzzy Expert System (FES) model tailored for determining optimal COQ in commercial projects while accounting for project risks. Using the means of the Prevention - Appraisal - Failure model and utilizing data gathered from prior research and validated through official questionnaires and interviews, this study assigns weights to quality risks and allocates them to respective COQ components. Risks such as inadequate planning, coordination issues, construction errors, and delays are identified as influential factors affecting COQ. Our findings suggest that COQ in commercial projects typically amounts to approximately five percent of the project budget. Validation of the prototype Fuzzy Expert System achieved through application to five actual cases (including two for system familiarization and three for validation), demonstrates an accuracy rate of 95%. The prototype Fuzzy Expert System emerges as a valuable tool for COQ evaluation, serving as a reliable checkpoint for individuals with varying levels of expertise in the field.