Growth stresses in amorphous SiO2 scales formed during SiC fiber oxidation were calculated. A numerical method using Deal-Grove oxidation kinetics and shear-stress dependent SiO2 viscosity was used. Initial compressive stresses in SiO2 of ∼25 GPa from the 2.2× oxidation volume expansion rapidly relaxes. At >1200°C, viscous flow of amorphous SiO2 further relaxes stress to negligible levels. At 700° - 900°C, axial and hoop stress at the GPa level persist in SiO2 near the SiC-SiO2 interface. Radial expansion of the scale causes hoop stress to become tensile, and axial stresses are driven to tensile values by the Poisson effect. These tensile stresses can be >1 GPa for thick scales formed at lower temperatures on surfaces with high curvature. Approximate analytical expressions for growth stress are discussed. Effects of viscosity variation as well as other assumptions and limitations of the calculation method are discussed.