Current residence time distribution models for counter-rotating extruders are mainly developed for fully filled machines. However, in practical situations, these extruders operate under starved conditions. To overcome this missing link in the current models, this paper describes the average degree of fill in the situation of starvation. This degree of fill is determined by the length over which the extruder is fully filled and the degree of fill in the partially filled zone. The comparison of the experiments with the theory shows that the existing theory underestimates the length of the fully filled zone by about 30%. The degree of fill in the partially filled zone is well predicted by taking into account all non-pressure driven leakage flows.