Process flexibility is important to ensure that an operation can be both profitable and manipulative to changes in internal or external process parameters. A new flexibility metric (FIV) is defined based on the hypervolume ratio of the feasible region and the region containing all combinations of expected uncertain parameters. The FIV evaluation requires a hypervolume determination which may not be straightforward for multidimensional systems or systems with nonconvex feasible regions. Thus, an auxiliary vector approach is developed to estimate the feasible space's size. Compared to the existing methodologies, this new method is able to provide a reliable and facile flexibility evaluation. In addition, this approach can also offer indications of the possible future changes to enhance the overall flexibility. This information is useful for making reasonable and rational engineering decisions to retrofit the designs.