Powder refinement in a planetary mill (Retsch PM 400-MA) is investigated experimentally and analyzed using discrete element modeling (DEM). Refinement is defined here as the average size of the individual components in a composite powder. The specific milling dose, defined as the product of charge ratio and milling time, is used as an experimental parameter to track the progress of the material refinement. This parameter is determined experimentally for reactive milling of boron and titanium powders, for which the time of initiation of a self-sustained reaction producing titanium diboride is measured under different milling conditions. It is assumed that the reaction is always initiated when the same degree of structural refinement is achieved. The DEM calculations established that the milling balls primarily roll along the milling container's perimeter. The inverse of the rate of energy dissipation resulting from this rolling motion is used as the DEM analog of the specific milling dose. The results correlate well with experimental observations. The usefulness of the DEM model is established in predicting the milling conditions for varied process parameters for mechanical alloying or reactive milling performed in a planetary mill.