Primary metallurgical plants 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 the determination of the mass of carry-over slag. Consequently, in literature several approaches applied in the past have been published. However, their area of application is usually limited to steel grades and production facilities, which were used for the development. Therfore, a new calculation model was developed within a bachelor thesis at Montanuniversitaet Leoben (Chair of Ferrous Metallurgy) in cooperation with RHI AG and voestalpine Stahl GmbH. This paper gives an overview on the modelling approach and development process. The focus is on the results of an extensive evaluation with industrial process data.