Although refinement of the as-cast grain size of magnesium alloys by Zr is well established commercially, little research has been undertaken to optimize this refinement technology and to quantify the relative performance of different Mg–Zr master alloys. The performance of Mg–Zr master alloys was found to be related to (1) the number density of Zr particles between 1 and 5 µm in size where the master alloy with the largest number density of these particles exhibits the best refinement, and (2) the alloy's growth restriction factor (Q) where a linear relationship between grain size and 1/Q, was found to exist for each master alloy. An equation for predicting grain size based on the Interdependence model was developed. Further, the Interdependence Model was improved to be able to cater for an increasing Zr particle number density with increasing values of Q.