Primary metallurgical aggregates and especially basic oxygen furnaces are usually equipped with slag-retaining systems to control slag carry-over. The amount of carry-over slag is an indicator for the efficiency of these devices. A quantitative evaluation of these systems requires a determination of the mass of carry-over slag. Therefore, in literature several approaches applied in the past are published. However, their area of application is generally limited to steel grades and production facilities, which were used for their development. Thus, within a bachelor thesis at Montanuniversitaet Leoben (Chair of Ferrous Metallurgy) a new calculation model with industrial process data was developed in cooperation with RHI AG and voestalpine Stahl GmbH. This paper gives an overview on the modelling approach and calculation results gained with data from voestalpine Stahl GmbH.