A formulation for defect volumes is given in the framework of lattice statics methods of Kanzaki. The harmonic theory of static lattice distortions is extended to include the effects of the change in the mean lattice parameter of the crystal as a result of introduction of defects. It is found that the above modifications enable the theory to give a correct description of vacancy formation volumes in ionic solids. Fractional relaxation volumes of formation of Schottky defects in some ionic crystals have been calculated and compared with latest experimental data. It is seen that the volume is quite sensitive to the displacements and electronic dipole moments of the ions in the neighbourhood of the vacancies. It is observed that the volume gets a predominant contribution from long range interactions.