Formation energies of nitrogen (N)/vacancy (v) monomers, N–N and N–v dimers and N–v complexions in an ideal face-centered cubic (fcc) lattice of iron are obtained via DFT calculations. Based on a thermodynamic model, the occupancies of various complexions and the vacancy concentration as a function of the chemical potential of N and temperature T are predicted. We found that increasing the chemical potential or content of N can increase the vacancy concentration considerably, e.g. compared with the case without nitrogen, increasing the N content to 10 at% at 750 K can increase the vacancy concentration by about eight orders of magnitude when a saturated concentration is reached.