The correlation between ionic radii and the lattice constants of orthorhombic perovskites is examined and new empirical formulae for the relationship are derived. As for cubic/pseudocubic perovskites, the lattice constants are largely a function of A–X and B–X bond lengths rather than ionic charge. The average absolute relative error in the predicted a, b, and c lattice constants so derived is expected to be about 0.616%, 1.089%, and 0.714%, respectively; therefore, these formulae are in better agreement with experimental data than those derived by earlier researchers.