Channel Fill Length, defined as the length of fully filled regions in an extrusion screw, is gaining importance as a design parameter because of its implications for residence time distribution, distributive and dispersive mixing, and also process stability. Although models exist for theoretical estimation of fill length, it is difficult to apply these models for complex geometries and fluid rheology. Owing to the nonlinear relationship between fill length, geometry and operating conditions, it is time-consuming to use empirical methods to characterize fill length behavior in any given extrusion process. In this work, it was found that fill length exhibits a linear relationship with the ratio of percent fill to percent empty fraction. Two different sets of fill length data taken from literature, one set from a co-rotating, intermeshing geometry and the other from a counter-rotating, non-intermeshing geometry, were used to verify this relationship. This property of the extrusion process can be used by the researcher to quickly characterize fill length behavior over the entire operating range.