Powder filling is challenging to measure in industrial mills. In this paper, three estimation models of powder filling are evaluated. The first is built around the crescent-shaped load; the second is empirical while the third exploits residence time distribution. In terms of supporting data, total mass content, ball filling, slurry density, load orientation, power draw, and material transport through the mill were measured under various conditions. The residence time distribution was then characterised and used in computing powder filling. Measured powder filling was compared against predictions and possible sources of discrepancy were discussed.