This paper proposes a methodology based in fuzzy opinion under group decision making, to obtain a single index of sustainability and indicate the best actions to improve this index. This proposed methodology was implemented in a software system that provides managers with an important computational support to detect problems and implement actions to improve processes in all areas of the company, aiming to supersede sustainability benchmarks. The system can unify a company's sustainability indicators of the most distinct dimensions (environmental, economic, and social), in a single index, which represents an innovative approach to sustainability evaluation.