Many European countries claim the struggle against wage inequality between men and women to be one of their priorities. Quantifying the real effects that should have been observed if such wage equalization was achieved remains to be done. A Shapley decomposition of the wage inequality allows to determine the a priori impact of a policy that removes the pay gap between men and women for a given age and education level. It also explains how interactions between personal characteristics lead to the obtained result. This economic policy tool is then illustrated with the wage distribution data of the French Labor Survey.