To determine the lean level of an organization a methodology was developed. It was based on a qualitative assessment approach, including quantitative basis, whose development was supported using fuzzy logic. Recourse to the use of fuzzy logic is justified by its ability to cope with uncertainty and imprecision on the input data, as well as, could be applied to the analysis of qualitative variables of a system, turning them into quantitative values. A major advantage of the developed approach is that it can be adjusted to any organization regardless of their nature, size, strategy and market positioning. Furthermore, the proposed methodology allows the systematically identification of constraint factors existing in an organization and, thus, provide the necessary information to the manager to develop a holistic plan for continuous improvement. To assess the robustness of the proposed approach, the methodology was applied to a maintenance and manufacturing aeronautical organization.