To establish the reason for the large voltages generated by ultrathin Mn3O4 under stress, and their superior performance over bulk Mn3O4, we used DFT to calculate the coupling of electric polarization to strain and strain gradients in Mn3O4. Typically, piezoelectric materials develop voltages upon application of stress. A necessary condition for piezoelectric materials is the lack of inversion symmetry; however, we find that both bulk Mn3O4 and the two low surface energy (101) slabs with octahedral and tetrahedral termination are centrosymmetric. Centrosymmetric materials can develop voltages due to the coupling between electric polarization and strain gradientsthat break the inversion symmetry; this linear coupling is termed as flexoelectricity. More recently, it has been proposed that flexoelectric charges arising from strain gradients on nanoscale features leading to triboelectric charge separation. Therefore, we calculated the flexoelectric coupling constants in monolayer and bilayer Mn3O4 and compared them with bulk Mn3O4. The flexoelectric tensor is a rank-4 tensor that relates strain gradient to polarization via Equation 1: