How do I calculate the size of steel beam I need?

First calculate the total UDL (uniformly distributed load) from the floor area loading multiplied by the supported width. Then calculate the bending moment using M = wL²/8 (for simply supported beams with UDL). Finally, select a beam with a moment capacity greater than the calculated bending moment from the steel section tables. This calculator uses S275 grade steel.

M = wL²/8 is the formula for the maximum bending moment of a simply supported beam with a uniformly distributed load. M is the moment in kNm, w is the load in kN/m, and L is the span in metres. This is the most common loading condition for domestic beams supporting floors or roofs above. Point loads require different calculations.

Do I need a structural engineer for a steel beam?

Yes, Building Regulations require structural calculations by a competent person (usually a chartered structural engineer) for all structural alterations including steel beams. This calculator provides indicative sizing only — it does not account for lateral torsional buckling, shear, deflection limits, web bearing, or connection design. Always get professional calculations.

What loading should I use for a domestic floor?

For domestic floors (BS EN 1991-1-1): imposed load = 1.5 kN/m² and dead load typically 1.0-2.0 kN/m² depending on floor construction. These are factored by partial safety factors (1.35 for dead, 1.5 for imposed) for ultimate limit state design. Roofs use lower imposed loads (0.75 kN/m²). Always verify loads with your structural engineer.

How much does a steel beam cost?

Steel beams in the UK typically cost £2.00-3.50 per kg for the steel itself, depending on size and supplier. A common 203x133x25 UB at 4m span weighs 100 kg and costs approximately £250-350 for materials. Add £500-2,000+ for installation including padstones, temporary support and making good. Delivery and crane costs are additional.

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Steel Beam RSJ Calculator — UDL, Moment & Beam Selection

Calculate UDL from floor loading, bending moment and select a suitable S275 steel beam from moment capacity tables.

Beam Span

Clear span between supports

Supported Floor Width

Width of floor supported by beam

Loading Type

Imposed: 1.5 kN/m², Dead: 1.5 kN/m²

Steel Price

How We Calculate This

This calculator determines the required steel beam size by calculating the bending moment from floor loading and selecting a suitable Universal Beam section.

Load calculation

UDL (kN/m) = (Dead Load × 1.35 + Imposed Load × 1.5) × Floor Width
Bending Moment M = wL² / 8 (kNm)
For simply supported beams with uniformly distributed loading.

Beam selection

The smallest S275 Universal Beam with a moment capacity exceeding the calculated bending moment is suggested. This is indicative only — a structural engineer must verify for deflection, shear, LTB and connections.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.