A combination of experiments and theoretical calculations is used to develop an atomic-scale model of the grain boundary potential in perovskite oxides. More specifically, pristine 8degrees and 58degrees  tilt grain boundaries in SrTiO3, which can be regarded as model systems for all cubic perovskite systems, are examined by Z-contrast imaging and electron-energy-loss spectroscopy. Based on results obtained from these systems, distance-valence least-square analysis and multiple-scattering calculations are used to determine the density of grain boundary states for the 8degrees and 58degrees grain boundaries, respectively. To compute the grain boundary potentials, the Thomas-Fermi approach of screened charges and the classical Schottky model is used. The validity of both models for various perovskite oxide grain boundary configurations is discussed, and the appropriate grain boundary potentials are compared with previously reported data.