What is the section modulus (Sx / Zx) and why does it matter?

The section modulus measures how efficiently a cross-section resists bending moment. Sx (elastic section modulus, Wel) is used for elastic design — maximum bending stress = M/Sx. Zx (plastic section modulus, Wpl) is used for plastic design — plastic moment = Zx × fy. For a given steel grade, a profile with higher Sx/Zx can resist more bending with less material. For example, an IPE 300 has Zx ≈ 628 cm³ giving plastic moment Mpl ≈ 157 kN·m in S250 steel. When choosing a beam profile, you compare required Z (from Mreq/fy) to available Z in the catalog, then pick the lightest profile. CalcSteel shows Sx, Zx, Iy, Iz and all section properties on each profile page.