We have carried out local atomic orbital basis first-principles total-energy calculations of (001) SrTiO3 (STO) surfaces with a half unit cell high step. The thermodynamic analysis shows that an SrO-terminated surface (A-site layer) has the lower surface energy under most chemical environments. We introduce a method to estimate the step energy for a SrTiO3 surface with a step based on thermodynamic considerations. We show that an STO surface with a half unit cell high step prefers an oxygen terminated step edge under oxygen rich conditions and an Sr- or TiO-terminated step edge under oxygen deficient conditions. However, under the majority of chemical environments the edge termination is mixed, and it is the stoichiometry that drives the terrace termination.