Simply put, the tonnage factor, in mining, is the ratio of the volume of material excavated (e.g., in cubic feet or cubic meters) to the weight of that material (e.g., in tons or tonnes). The tonnage factor is therefore the inverse of the material (bank) density.

Example: If 1400 cubic feet of material was excavated and the total weight of said material was 100 tons, the tonnage factor would be 14 ft3/ton (density = 143 lb/ft3).