The viscosity of mould slags is one key parameter in the continuous casting process of steel. Its measurement using a viscometer is costly and time and consuming. Therefore, beside viscosity modelling, an inclined plane test to determine viscosity was proposed by several authors. The task of this study was to find out, whether this method is suitable for quality control of mould slags, or not. Therefore, an inclined plane test device (IPT) was constructed and operated. At the same time, the slags were investigated by a viscometer. In total, 80 different slag compositions were investigated. Several options to represent the viscosity as a function of the ribbon length have been studied, and a suitable relation is proposed. It serves for detailed statistical studies to assess the error of IPT. While the relation between viscosity and length shows a satisfactory degree of determination (e.g.>0.97) in this study, this still does not ensure sufficient accuracy of individual applications of IPT. As the slag undergoes cooling during the test, also the viscosity change in dependence of the temperature contributes to the result, not only the viscosity of the initial impact temperature. This raises the unexplained variance of the result.