A simulation method for heterogeneous nucleation has been developed that enables the prediction of the resulting grain size in Mg-alloys as a function of the inoculant particle size distribution, cooling rate, alloy constitution and volumetric content of inoculants. Experiments with (SiC)P and (Al4C3)P have been already successfully performed and verified the model assumptions. In view of the considerably smaller lattice mismatch between ZrB2 and Mg compared to SiC and Al4C3, respectively, (ZrB2)P should act even more as potent inoculant for grain refinement. However, the larger density of ZrB2 leads to sedimentation of the inoculants that greatly alters the particle size distribution and therefore the efficiency of grain refinement. A theoretical estimate for the final grain size under consideration of the settling effect has been performed and will be discussed in view of experimental results.