# Steel Beam Span Calculator

The depth is not looked up in a table — it falls out of the deflection limit.

- Page: https://jianzhujisuan.com/en/steel-beam-span
- Category: Steel Design (https://jianzhujisuan.com/en/c/steel-design)
- Chinese version: https://jianzhujisuan.com/zh/steel-beam-span.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Span L 6 m, Uniform load w 20 kN/m, Modulus E 210000 MPa, Yield strength fy 250 MPa, Deflection limit L/360 (with finishes)): **Required inertia 9643 cm⁴**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Span L | number | 6 | m |  |
| Uniform load w | number | 20 | kN/m |  |
| Modulus E | number | 210000 | MPa |  |
| Yield strength fy | number | 250 | MPa |  |
| Deflection limit | select | L/360 (with finishes) | — |  |

## Formula

- **Allowable deflection**: δallow = L/n
- **Required inertia**: Ireq = 5wL⁴/(384E·δallow)
- **Maximum moment**: M = wL²/8
- **Required modulus**: Wreq = M/fy
- **Reaction**: R = wL/2

## Worked example

6 m span at 20 kN/m with E 210000 MPa, fy 250 MPa and L/360: 16.7 mm allowable, 9643 cm⁴ required, 90.0 kNm maximum moment, 360 cm³ of section modulus and 60.0 kN reaction.

## Questions

### How far can a steel beam span?

There is no fixed limit; it depends on load and allowable deflection. A rough start is a span-to-depth of 20, economical between 18 and 25, so 300 mm deep at 6 m. The real calculation is the deflection sum, Ireq = 5wL⁴/(384Eδ) — span enters as the fourth power, so doubling span needs 16 times the inertia. That is why long spans go to trusses or cables: a solid web beam becomes impossibly heavy.

### What deflection limit should I use?

L/250 for general floors, L/360 where finishes or partitions sit above, L/400 to L/500 for sensitive equipment or glazed facades, and as loose as L/180 for roof purlins — provided you check separately that drainage still falls the right way. These are live load limits; long-term dead load deflection is additional. Steel suffers little from that compared with concrete creep and shrinkage, but a composite slab changes the picture.

### Why is deeper better than thicker?

Because inertia goes with depth cubed, I = bh³/12 for a rectangle, and only linearly with width. Going from 300 to 400 mm depth multiplies inertia by 2.37, while thickening a flange from 10 to 13 mm adds about 30% inertia for 30% more weight. Optimising a steel beam therefore always means going deeper: the same tonnage in a deep slender section outperforms a shallow stocky one. The limits are lateral stability and local buckling — tall webs need stiffeners and thin flanges buckle locally.

### How much does composite action help?

A great deal. Once the slab is connected to the beam by shear studs the concrete becomes the compression flange and the steel works mainly in tension, raising the effective inertia by 1.5 to 3 times depending on slab depth and stud count. That is why real steel beams are often a size smaller than a bare-steel check suggests. The price is three extra checks: stud shear, the bare beam carrying the wet concrete, and the internal forces from slab shrinkage.

## Related tools

- [Section Properties Calculator](https://jianzhujisuan.com/en/section-modulus): Inertia governs deflection, section modulus governs strength — they are not interchangeable.
- [Fillet Weld Strength Calculator](https://jianzhujisuan.com/en/fillet-weld): A 6 mm leg is not a 6 mm throat — capacity follows the 4.2 mm throat.
- [Base Plate Design Calculator](https://jianzhujisuan.com/en/base-plate): Thicker is not the answer — check the bearing area first, then the cantilever.
- [Column Buckling Calculator](https://jianzhujisuan.com/en/column-buckling): Half the effective length means four times the Euler load — but the design resistance differs by less, and how much depends on slenderness.

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