A compact gains strength in sintering through low-temperature interparticle bonding, followed by further strength contributions from high-temperature densification. On the other hand, thermal softening substantially reduces a compact's strength at high temperatures. Therefore, the in situ strength during sintering is determined by the competition among interparticle neck growth, densification, and thermal softening. Distortion in sintering occurs when the compact is weak. Most strength models for sintered materials are semi-empirical relations based on the sintered fractional density. These models do not include microstructure or sintering cycle parameters; thus, they do not provide guidelines for thermal cycle design to improve compact dimensional control. A strength evolution model is derived which combines sintering theories and microstructure parameters, including interparticle neck size, solid volume fraction, and particle coordination number. The model predicts sintered strength and when combined with thermal softening gives a good prediction of in situ strength. The validity of the model is verified by comparison to experimental data for sintered and in situ strength of bronze and steel powders.