A failure model for die-cast magnesium alloys is formulated based upon a previously published critical strain model and the analysis of the stress concentration factor due to a spherical void. The failure model predicts the fracture strain during uniaxial tensile loading from the strain-hardening coefficient of the magnesium alloy and the area fraction of porosity due to a macropore in the microstructure. It was determined that this model predicts the fracture strain of 10 tensile samples with holes of different diameters drilled through the cross-section with reasonable accuracy. The predictions are further shown to agree with literature data for three magnesium-based casting alloys.