The mechanical strength of solid-state electrolytes is crucial for applications in solid-state batteries (eg block lithium dendrites and withhold the stress due to volume expansion of the electrode). We determined the flexural strength of doped Li7La3Zr2O12 (Li7La2.75Ca0.25Zr1.75Nb0.25O12, LLCZN) garnet type electrolyte as 8.6 +/- 1.5 MPa to 51.5 +/- 4.8 MPa as functions of porosity and grain size. Based on the experimental results, a series of theoretical models were established to obtain empirical equations to study the effects of porosity and grain size. Our further experiments demonstrate that the empirical equations can be used to predict the flexural strength of garnet type oxides based on its microstructure (porosity and grain size). Optimization of the grain size of fully dense garnet oxide can increase the strength up to 106.0 +/- 11.6 MPa (tape casting-sintering method with a grain size of 2.8 mu m).