Abstract Most existing formulations for determining shear strength due to web-post buckling by shear (WPBS) in castellated steel beams depend on many variables, are not applicable to fire situation or do not have a normative approach. In order to provide a simpler and more practical formulation, also applicable to the fire situation, this study presents a semi-empirical formulation based on numerical results of validated models, considering the hypothesis of uniform temperature distribution along the profile and the analogy between the strut model and the compressed diagonal of a web-post subject to WPBS. This semi-empirical model is based on the formulation of EN 1993-1-2:2005 and ABNT NBR 14323:2013 to obtain the resistance of compressed members at elevated temperatures and it is applicable to Litzka, anglo-saxon and Peiner type castellated beams.