What is M = wL²/8 and when does it apply?

M = wL²/8 is the bending moment formula for a simply supported beam with a uniformly distributed load (UDL). 'w' is the load per metre (kN/m), 'L' is the clear span in metres. It applies to most domestic floor beams, lintels, and similar simply supported conditions. For point loads or cantilevers, different formulae apply.

What deflection limit should I use for a steel beam?

The standard deflection limit for steel beams is span/360 under imposed load only. For beams supporting brittle finishes (plaster ceilings), use span/360. For beams supporting masonry, span/325 may be required. For industrial buildings without sensitive finishes, span/200 may be acceptable. BS EN 1993-1-1 refers to the National Annex for specific limits.

What steel grade should I specify?

S275 is the standard grade for most UK structural steelwork — it has a yield strength of 275 N/mm². S355 (yield 355 N/mm²) is stronger and used where weight or depth must be minimised. S355 costs slightly more per tonne but you often use less steel, so it can work out cheaper overall. Most steel stockholders carry both grades.

How much extra weight should I allow for connections?

A typical allowance for connection plates, bolts, cleats, and stiffeners is 10-15% of the main steel weight. For simple beam-to-column connections, 10% is usually adequate. For moment connections, portal frame haunches, or complex fabrications, allow 15% or more. Your steelwork fabricator can give a precise figure from their shop drawings.

Can I use this calculator for a beam supporting a wall above?

Yes, but you need to determine the correct UDL first. For a wall above a beam, if the wall height exceeds 0.6 × span, only the triangular load within a 60° load spread is carried by the beam. Use the load from the supported triangle of masonry plus any floor loads. For padstone and bearing details, see our RSJ Installation Calculator.

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Universal Beam Calculator — UB Size Selection & Load Check

Calculate the required universal beam size for a simply supported beam under uniformly distributed load. Checks moment capacity and deflection per BS EN 1993.

Clear Span

Distance between supports

Uniformly Distributed Load (UDL)

Total factored load per metre run

Steel Grade

S275 is standard for most UK structural work

Price Per kg (optional)

Enter your steel price per kg for a cost estimate

How We Calculate This

This calculator sizes a universal beam (UB) for a simply supported condition with a uniformly distributed load (UDL), following BS EN 1993-1-1 (Eurocode 3) principles.

The formulas

Bending moment: M = wL²/8 (kNm) for UDL on simply supported beam
Deflection: δ = 5wL⁴ / (384EI) where E = 205,000 N/mm²
Deflection limit: L/360 (standard) or L/325 (supporting masonry)

Selection process

The calculator works through the Tata Steel Blue Book section catalogue from lightest to heaviest, finding the first beam whose moment capacity exceeds the applied moment AND whose deflection is within the specified limit. It then reports the utilisation ratio, weight, and deflection check.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.