The elastic section modulus S divides the second moment of area I by c, the distance from the centroid to the extreme fibre, so that bending stress at the surface is simply the moment divided by S. For a rectangle it works out to Sx = bh²/6 — a 200 × 300 mm section gives 3.0 × 10⁶ mm³. For unsymmetric shapes like angles, channels, and T-sections the centroid sits off-centre, and the governing modulus uses the larger fibre distance, since that face sees the highest stress.
There are actually two section moduli. The elastic value S marks first yield at the extreme fibre; the plastic section modulus Z assumes the entire cross-section has yielded and is always larger. Their ratio is called the shape factor, and plastic design methods lean on Z where the elastic approach uses S.
Steel tables are organised around this property: AISC W-shape tables list the strong-axis elastic modulus as Sx in in³, while the European IPE, HEA, and HEB tables print the identical physical quantity as Wel,y in cm³ — Europe labels the strong axis y-y rather than x-x. Shortlisting a beam typically starts by scanning the Sx column for a value that covers the design moment, then confirming the pick with full bending, shear, deflection, and lateral-torsional buckling checks.