A 3m 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.
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 3m calculation, not a beam recommendation
Suppose a simply supported 203×133×25 UB spans exactly 3m 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 2.58 mm deflection. A selected L/360 comparison target would be 8.33 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.
A short opening can still collect a large load
Measure the floor width feeding the beam rather than judging the load by the opening alone. If joists span toward the opening from two sides, the load arrangement may differ from a one-sided floor. A masonry wall, chimney or another beam above can introduce loads that a simple floor-area estimate misses. Give the engineer the joist directions and support locations before deciding that a 3m opening is a light-duty case.
Change the load while keeping the span fixed
Keep the 3 m support-centre span and section fixed for a second illustrative calculation. Entering 6 kN/m total load instead of 12 kN/m gives a reaction of 9 kN at each support, a maximum moment of 6.75 kNm and about 1.29 mm deflection. Doubling the uniform load doubles each of these outputs in this linear model.
| Total uniform service load, including beam weight | Reaction at each support | Maximum moment | Calculated deflection |
|---|---|---|---|
| 6 kN/m | 9 kN | 6.75 kNm | 1.29 mm |
| 12 kN/m | 18 kN | 13.50 kNm | 2.58 mm |
The numbers illustrate sensitivity to a stated input. They are not alternative design loads to choose from. The actual floor construction, loaded width and other supported elements determine the loads for the project.
What to send with a small-opening enquiry
A short opening can be described clearly on one marked-up plan. Show the opening location, joist directions and any member framing into the wall. Add photographs of both ends and explain whether the room above changes use or layout. Ask the engineer what concealed details need investigation.
Keep the brief specific: “3 m clear opening in this wall, with these floors above” provides more useful context than “a beam for a 3 m span”. It also gives the designer a chance to explain whether a simply supported uniform-load model is appropriate at all.
If the support-centre span is actually 3.2 m, the 12 kN/m example gives about 3.33 mm deflection rather than 2.58 mm. Use the dimension established for the model, not a rounded measurement selected because it matches an article title.
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.