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Steel Beam Span Calculator

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

Also known as steel beam span · beam span calculator · steel beam sizing

Quick answer

With the default inputs: Span L 6 m · Uniform load w 20 kN/m · Modulus E 210000 MPa etc.Required inertia 9643 cm⁴

Use your own dimensions ↓

Inputs

kN/m
MPa
MPa

Result

Required inertia
9643cm⁴
Allowable deflection16.7mm
Required Ix9643cm⁴
Maximum moment M90.0kNm
Required Wx360cm³
Suggested depth (L/20)300mm
End reaction R60.0kN
Inertia at L/25013886cm⁴
Inertia per kN/m482cm⁴ per kN/m
How this is worked out

A steel beam is usually sized by deflection, not strength — the opposite of concrete. Steel at 250 MPa against concrete at 30 MPa means the strength-sized section is often too flexible: it bounces underfoot and cracks the finishes above. At 6 m and 20 kN/m the strength check needs 360 cm³ of section modulus while L/360 needs 9643 cm⁴ of inertia, which is a section one or two sizes larger. Inertia goes with depth cubed, so 300 to 400 mm raises it by 2.37 times for 33% more weight — deeper always beats thicker. L/20 is a reasonable starting point for steel and L/12 to L/15 for concrete, but these are rules of thumb; the size still comes from the deflection sum. Remember self weight, typically 5 to 15%, and feed it back for long spans.

Next up: Section Properties Calculator · Fillet Weld Strength Calculator

Formula

Allowable deflectionδallow = L/n
Required inertiaIreq = 5wL⁴/(384E·δallow)
Maximum momentM = wL²/8
Required modulusWreq = M/fy
ReactionR = wL/2
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.

Basis

FAQ

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.

What to calculate next

These jobs are done back to back on site — follow the steps to work through the whole sequence.

Steel: span to connectionStep 1 of 4

  1. 1Steel Beam Span Calculator
  2. 2Section Properties Calculator
  3. 3Column Buckling Calculator
  4. 4Fillet Weld Strength Calculator

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Steel Beam Span Calculator. 2026-10-25. https://jianzhujisuan.com/en/steel-beam-span

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.