A 5m opening does not determine an RSJ size. The answer depends on what bears on the beam, the load width, lateral restraint, support condition and permitted movement. Earlier generic section recommendations have been removed because they did not establish those conditions.
Measure the right span
Record the clear opening and both supports separately. Your engineer determines the effective span; our calculators use support-centre distance. The ordered steel length is another dimension, set by the bearing or connection detail. Do not use a calculator input as a cutting instruction.
A 5m calculation, not a beam recommendation
Suppose a simply supported 203×133×25 UB spans exactly 5m between support centres and carries a total uniform service load of 12 kN/m including its own weight. With Iₓ = 2340 cm⁴ and E = 210,000 N/mm², the elementary model predicts 19.87 mm deflection. A selected L/360 comparison target would be 13.89 mm. Neither number checks strength, buckling, bearings or fire resistance. The assumed 12 kN/m is not a default domestic design load.
The calculation is δ = 5wL⁴/(384EI), using N and mm. You can reproduce it in the deflection calculator and compare sections in the beam size tool.
Source: British Steel Universal Beams datasheet, 21 July 2026. The shared section table and methodology lists the exact inputs used by our tools. These properties are not safe load ratings.
Restraint and ceiling clearance are separate constraints
A proposed 5m beam may appear acceptable in a deflection comparison while still needing a different arrangement for stability or connections. Tell the engineer how the compression flange would be restrained and whether the beam must fit within a floor zone. Do not assume an enclosure supplies structural restraint. If headroom requires a shallower member, have the complete design reconsidered instead of selecting a wider-looking section from a photo.
Prepare a useful engineering brief
Provide measured plans, joist directions, floor and wall construction, storeys above, any roof reactions, proposed opening dimensions and photographs of the support locations. Flag chimneys, columns or other concentrated loads. Explain ceiling-height constraints, access for delivery, finish requirements and the jurisdiction for approvals.
Ask for a complete drawing and calculation package identifying the beam, grade, supports, connections and temporary works responsibilities before obtaining fabrication quotes.
What still needs a design check
A calculated deflection is only one part of a design. Load combinations, steel grade, buckling restraint, shear, connections, bearings, masonry, vibration and fire resistance also matter. Point loads, cantilevers and continuous beams require a different model. The engineer must establish the effective span and loads from the actual building; a clear opening measurement is not sufficient.
Structural work must be designed by a chartered structural engineer and receive the applicable Building Control approval. This article has not received an independent engineering review.