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Steel Beam Selection Calculator

Enter the span, uniform load, and allowable stress to size the beam.

How it is calculated

The tool uses US customary units and a simply supported beam carrying a uniform load over the whole span. Inputs: span L in feet, load w in pounds per foot (lb/ft, including the beam's own weight if you want it counted), and allowable bending stress σ in psi, which defaults to 24,000 if left empty. Maximum moment: M = w × L² ÷ 8 in lb·ft. It is converted to lb·in by multiplying by 12, and the required section modulus is S = M(lb·in) ÷ σ in in³. Pick a standard shape whose tabulated S is at least this value. All inputs must be above zero. Deflection, shear, lateral buckling and load factors are not checked.

Worked example

A beam spans 20 ft and carries 1000 lb/ft with an allowable stress of 24,000 psi. Step 1: M = 1000 × 20² ÷ 8 = 1000 × 400 ÷ 8 = 50,000 lb·ft. Step 2: in inch-pounds, 50,000 × 12 = 600,000 lb·in. Step 3: S = 600,000 ÷ 24,000 = 25.00 in³. Any shape with a section modulus of 25 in³ or more satisfies the bending check for these inputs, before deflection and other checks.

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Common mistakes

  • Entering the total load on the beam instead of the load per foot. The formula wants lb/ft, so a 20,000 lb total over 20 ft is 1,000 lb/ft.
  • Mixing units, for example a span in metres or stress in MPa. The result only makes sense with feet, lb/ft and psi.
  • Entering a yield stress rather than an allowable stress. The allowable value is lower, and using yield removes the safety margin.
  • Treating the section modulus as the final answer. Deflection, shear and lateral stability can govern, and beams with point loads or other support conditions need a different moment.

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.

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Comments (1)

  • Editorial team

    The load you enter should already include the beam's own weight, which you only know after choosing a shape. Try it as two passes: at 500 lb/ft over 20 ft the tool shows 12.50 in³, and adding 20 lb/ft for a trial beam raises it to 13.00 in³. If the first pick still clears the new figure, you are done with bending. This is only a first check, so a structural engineer should confirm. What span-to-depth rule do you use for deflection?

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