Scandia-zirconia system has great technological interest as it has the highest ionic conductivity among doped zirconia ceramics. However, polymorphism is the most important limiting factor for application of this material. Considering that there is a strong grain size dependence on phase transitions in this class of materials, mapping out the stable polymorph as a function of grain size and composition may lead to more efficient compositional design. In this article, the phase stability of zirconia-scandia nanocrystals was investigated based on experimental thermodynamic data. Exploiting recent advances in microcalorimetry, reliable surface energy data for five polymorphs of scandia-zirconia system: monoclinic, tetragonal, cubic, rhombohedral and are reported for the first time. Combining surface energy values with bulk enthalpy data obtained from oxide melt drop solution calorimetry allowed us to create a predictive phase stability diagram that shows the stable zirconia polymorph as a function of composition and grain size of the specimen within the range of 0-20mol% scandia.