Atomic scale computer simulations, based on density functional calculations, are used to investigate the variation of lattice parameter and bulk modulus with nitrogen deficient non-stoichiometry in titanium and zirconium nitrides. Results assuming a simple distribution of nitrogen vacancies best reflect the remarkably small observed changes in lattice parameter over the whole stoichiometry range for both materials. These are facilitated by small charge transfer to the cations immediately surrounding the nitrogen vacancy and small accompanying lattice relaxations. Conversely variations in bulk modulus are considerable but can be correlated, via a simple function, to the change in materials density.