The increasing application of numerical simulation in metal forming field has helped engineers to solve problems one after another to manufacture a qualified formed product reducing the time required. Accurate simulation results are fundamental for the tooling and the product designs. Many factors can influence the final simulation result like for example a suitable yield criterion . The wide application of numerical simulation is encouraging the development of highly accurate simulation procedures to meet industrial requirements. Currently, industrial goals of the forming simulation can be summarized in three main groups : time reduction, costs reduction, increase of product quality. Many studies have been carried out about: materials, yield criteria [3, 4, 5] and plastic deformation [6, 7, 8], process parameters [9, 10, 11] and their optimization, geometry modification of the stamped part to evaluate if process responses modifications are required, reaching the goal to perform a virtual tryout of the whole deformation process . In this paper proper metal forming numerical model and experimental analysis have been developed in order to foresee process responses in the case of sheet hydroforming technology. The interactions among the process performances and its variables are the most interesting aspects of the research because their knowledge means the possibility to drive the process feasibility which can be represented by the absence of ruptures and/or wrinkles in the stamped component. This paper analyzes the sheet thickness variation during the hydroforming process, according to a specifically defined “shape ratio”, useful to characterize product’s geometry. The latter is an hydroformed product characterized by a rectangular characteristic section with a drawing depth of 150mm, obtained by a hydroforming operation on a blank having a hexagonal shape. The physical and numerical experimentations were carried out on multiple geometries, different each others in punch radius and die radius, and on multiple materials, steel FeP04 (with a thickness of 1mm and 0,7mm) and Aluminum Al6061 (with a thickness of 0,7mm). The numerical simulation, validated by the experimental investigations [13,14], allowed to define a relationship, specific for sheet metal hydroforming, between the defined shape ratio and the key performance indicator, that is the percentage reduction thickness measured on specific areas of the formed part. The development of numerical models with an high level accuracy could give the real possibility to evaluate process feasibility with different combinations of geometrical and materials parameters without, at the first glance, simulation but only analyzing the specific curves (y = percentage reduction thickness, x = shape ratio).