One of the key values describing the rheology of a polymer is the shear viscosity. If one wants to simulate the injection molding process the viscosity depending on the shear rate and the temperature is mandatory to characterize a polymer. But how can this value be measured? Unfortunately, there is no machine available to directly measure the viscosity. Usually these values are calculated indirectly. A specific deformation of the material at a certain temperature requires a definite force. This force (pressure / momentum) can be measured. Based on several assumptions, corrections and theoretical models the viscosity at different shear rates can be calculated. There are different techniques to measure this force. The most common way to get the viscosity for polymers at high shear rates is the High Pressure Capillary Rheometer (HKR). The material is pushed through a capillary with different velocities and the pressure is measured. Based on the assumption that the Newtonian and the shear thinning viscosity are equal at a specific position in the capillary the viscosity can be calculated. Several corrections (Bagley, Mooney or Weisenberg Rabinowitsch …) can be applied to take into account certain effects like elasticity effects, wall slip or a non-Newtonian velocity profile. Other effects like the temperature increase are neglected. Since the viscosity cannot be measured directly, it is almost impossible to judge how accurate the results are. An interesting experiment was performed, using simulation software to calculate the pressure loss in an HKV. The simulated pressure curves can be compared with the measured curves which were used to calculate the viscosity. Several artificial experiments where performed virtually, taking into account different effects one after another to see the influence on the final results. The presentation will give an overview about the standard corrections for viscosity measurements. The influence of the different corrections on the calculat