Since necking in some novel processes such as hydroforming and incremental forming can occur at locations where in addition to the in-plane stresses normal and through thickness shear stresses are exerted on the sheet metal, the plane stress assumption is not proper for predicting forming limits in these processes. Consequently in this research, conventional Marciniak-Kuczynski formability model has been generalized to consider all possible loading conditions including normal and through thickness shear stresses. For this purpose, additional force equilibrium and geometrical compatibility have been supposed between groove and matrix and Newton–Raphson trend has been applied for solving final system of equations in the numerical model to calculate limiting strains. In order to validate the theoretical model, subsequent theoretical forming limit curves have been compared with published experimental data. It has been shown that the extended M–K model is in good compatibility with experimental results and increase in normal and through thickness stresses enhances forming limits.