Steel Beam Selection Calculator

About this calculator

Sizing a steel beam starts with finding the maximum bending moment it will experience. For a simply supported beam under a uniform load, that's M = w·L²/8, where w is the load per unit length and L is the span between supports — this is one of the most-used formulas in structural design because so many real beams (floor joists, roof purlins) approximate this loading case.

Dividing that moment by the material's allowable bending stress gives the minimum section modulus (S) the beam needs, which you then match against a steel W-shape or I-beam table (like AISC's) to pick an actual size. This calculator is a quick first-pass sizing check for structural engineers and engineering students before running a full code-compliance check with deflection, shear and lateral-torsional buckling limits.