This paper is concerned with the volumetric performance of enclosed screw conveyors with particular reference to the influence of vortex motion. Vortex motion arises as a result of internal friction, friction between the granular material and surface of the helical blade, and the infinitely variable helix angle of the helical flight from the outer periphery of the blade to the shaft. The vortex motion, together with the degree of fill, govern the volumetric efficiency and, hence, the volumetric throughput. An analysis of the vortex motion in vertical or steeply inclined screw or auger conveyors is presented. It is shown the vortex motion is characterised by the tangential component of the absolute grain velocity being substantially constant with the radial position of a point on the blade. On this basis, an expression for the volumetric efficiency is derived. The volumetric throughput can then be predicted. The paper also presents results from an experimental screw conveyor where it is shown that the analytical predictions correlate very closely to the measured results.