A subregular solution thermodynamic model was employed to calculate the stacking fault energy (SFE) in Fe–Mn–Al–C–Si steels with contents of carbon 0.2–1.6 wt.%, manganese 1–35 wt.%, aluminum 1–10 wt.%, and silicon 0.5–4 wt.%. Based on these calculations, temperature-dependent and composition-dependent diagrams were developed in the mentioned composition range. Also, the effect of the austenite grain size (from 1 to 300 μm) on SFEs was analyzed. Furthermore, some results of SFE obtained with this model were compared with the experimental results reported in the literature. In summary, the present model introduces new changes that shows a better correlation with the experimental results and also allows to expand the ranges of temperatures, compositions, grain sizes, and also the SFE maps available in the literature to support the design of Fe–Mn–Al–C–Si steels as a function of the SFE.