Composition-structure relationships are needed for various applications, including lattice-matching for heteroepitaxy; however, a general model to predict lattice constants in defective perovskites is not yet available because the exact nature of A-site vacancies in perovskites remains largely unknown. In this study, it has been shown experimentally via Le Bail refinements of x-ray diffraction data that such vacancies in (Pb1−3xLa2x□x)TiO3 and (Pb1−3xLa2x□x)(Zr0.6Ti0.4)O3 have an effective size due to both Coulombic repulsion of coordinating oxygen ions and bond relaxation. For the first time, cell volume can be predicted in this system from stoichiometry and published ionic radii data alone to within 0.2% accuracy within compositional range and. The model may be applied to other perovskite systems and eventually provide tailored properties (magnetic, dielectric, and other) based on improved structure predictions.