The Bagley correction is needed to calculate the inlet pressure drop when round hole capillaries are used. Near the inlet area of round hole capillaries, a pressure drop is resulting from convergent flow in the capillary inlet. In the same way a part of the pressure is also used for the outlet. The Bagley correction is supporting the separation of the viscose pressure drop and the inlet/outlet pressure drop. To determine the inlet/outlet pressure drop the pressure drop is being applied with different capillaries of the same diameter but different lengths and extrapolated to zero (Bagley plot).

For highly viscous materials (> 1 Pas), e.g. plastics and rubbers, the Bagley correction is the correction that will be applied first. After the Bagley Correction has been carried out the Rabinowitsch-Weissenberg correction can be carried out. This sequence must be adhered for physical reasons. The GÖTTFERT software WinRheo II is covering the order of the corrections in an automatic way.