The effects on the lattice parameter due to incorporation of high concentrations of donor impurities in SrTiO 3 are examined both experimentally and theoretically. Experimental lattice parameters were obtained from x-ray diffraction on La-doped films grown on bulk SrTiO 3 substrates with carrier concentrations up to 2 × 10 21 cm -3. The observed increase in lattice parameter is attributed to two causes: impurity-size and electronic effects. The latter can be attributed in part to the energy gain by lowering the conduction band upon volume expansion. These contributions are evaluated explicitly using hybrid functional calculations, with a net result in very good agreement with experiment.