On modern extrusion plants, polyolefins are generally processed using barrels that have a grooved feed section. Accurate calculation of the melt throughput is of decisive importance for the process engineering layout of these extruders. Up to the point at which the limit speed that gives rise to the conversion from solid friction to melt film friction is attained, the conveying capacity is determined by the feed zone that is filled with granules. This paper sets out a method of calculating the throughput for speeds below this limit, paying consideration to the melt flow components in both the screw channel and the grooves. The calculation method was verified experimentally with the aid of throughput measurements for different polyolefins. The results generally fall within a tolerance range of +/-10%.