Machining wear models are useful for the prediction of tool life and the estimation of machining productivity. Existing wear models relate the cutting parameters of feed, speed, and depth of cut to tool wear. The tool wear is often reported as changes in flank width or crater depth. However, these one-dimensional wear measurements do not fully characterize the tool condition when tools wear by other types of wear such as notching, chipping, and adhesion. This is especially true when machining difficult-to-machine materials such as titanium. This paper proposes another approach for characterizing tool wear. It is based on taking measurements of the retained volume of the cutting tool. The new wear characterization approach is used to demonstrate the progression of volumetric wear in titanium milling.