Abstract Effective ionic radii in crystals with higher concentrations of defects may considerably differ from tabulated values. For a number of perovskite-type oxides A 1− a A′ a B 1− b B′ b O 3± x (A, A′=rare earth, earth alkaline, B, B′=Al, Ga, In, Zr, Ce, Cr, Mn, Fe, Co, Mg) a calculation mode for average ionic radii of each sub-lattice is proposed on the basis of experimentally determined oxygen stoichiometries (vacancy concentrations) and unit cell volumes. The effect of the vacancies on the lattice expansion is considered. A two-dimensional radius diagram combined with Goldschmidt’s tolerance factors resulting from the effective radii represents the structure modifications of non-defective and cation vacant perovskite-type oxides. For anion vacant perovskite-type oxides a three-dimensional diagram with the axes r A– r B– r O was constructed. For more than 50 compositions of oxides the specific free volumes of the unit cell are correlated with the t-factors calculated from the effective ionic radii. Independently on structural modifications of the perovskite-type and of the defect-type a linear relation between V f,s and t was found. The approaching character of the calculation scheme is discussed, and the results are evaluated with regard to the gradually improved calculation modes of t-factors.