The formation energy of the coherent interface between the primary strengthening phase γ ′ ′ and γ matrix in Inconel 718 alloy is investigated using ab initio calculations. We begin by examining the interface energy of the ordered Ni/Ni 3 Nb system. A negligible interface energy (1 mJ/m 2 ) is obtained for the nonmagnetic state, which is explained by a nearest-neighbor layer interaction model. Allowing for spin polarization within both face-centered cubic (FCC) and D0<inf>22</inf> structures increases the interface energy to 181 mJ/m 2 . The strong magnetic dependence of the formation energy of the ordered Ni/Ni 3 Nb interface arises primarily from the different magnetic behavior of Ni in FCC and D0<inf>22</inf> phases. A detailed analysis of the site preference of minor elements in the γ ′ ′ phase shows that Fe and Cr occupy the Ni-site, while Al, Mo, and Ti tend to occupy the Nb-site. The coherent interface energy of the γ / γ ′ ′ interface is predicted to be 257 mJ/m 2 for the paramagnetic state and 255 mJ/m 2 for the ferromagnetic state. The closeness of these formation energies reflects a similar magnetic response from both phases. The sensitivity of the γ / γ ′ ′ interface energy to variations in the γ and γ ′ ′ compositions is also investigated. Only small variations are revealed for the reported composition intervals. Our predictions serve as input for further theoretical simulations and as a reference for experimental investigations.