For fatigue life estimation as well as for fatigue testing analysis of welded structures by local stresses approaches, the stress concentration factor (SCF) at the weld must be known. Full penetration butt welded joints are extensively used in steel structures. Some empirical formulas to estimate SCFs are available in the literature, but their ranges of application are not always clearly defined, as well as the hypotheses on their local geometry. In this study, notch SCFs at steel butt welded joints under uniaxial tension are investigated by means of 2D plane strain linear elastic finite element analysis. The weld profile is modeled with splines. A detailed definition of the geometry is given. The influence of parameters such as the plate thickness, the weld reinforcement, the flank angle, the weld toe radius, the weld length and the presence of linear misalignment is assessed. Finally new parametric equations to estimate the SCFs at steel butt welded joints are proposed.