In recent decades, there has been an increase in the usage of cold-formed steel sections as structural components in buildings. It is crucial that fire safety be included from the initial stages of the design process in such buildings. Post fire capacities of the structural elements also hold equal importance as they help determine their reusability. There has been limited research on the fire and post fire capacities of cold formed steel built-up beams. This work presents the numerical investigation of cold-formed steel (CFS) built-up box beams exposed to elevated temperatures and post fire conditions. This study will extend the scope of previous studies by including exposure to elevated temperatures up to 900 ºC, and evaluating the post fire capacities using experimentally evaluated mechanical properties, including different grades of steel and spans. Validated geometrical and material non-linear finite element models were used to predict the failure moments and collapse modes. Moment capacity results were compared with the predictions based on the direct strength method design equations.