A 4m 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 4m calculation, not a beam recommendation
Suppose a simply supported 203×133×25 UB spans exactly 4m 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 8.14 mm deflection. A selected L/360 comparison target would be 11.11 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.
Check whether the load is actually uniform
A 4m kitchen opening may receive a concentrated reaction from a member above. Spreading that force evenly across the span in a calculator changes the bending and deflection model. Record the force location and its source for the designer; do not turn a point force in kN into kN/m merely by dividing by four and assuming the result is equivalent. Our online tool cannot represent that mixed loading.
Compare uniform and concentrated loading
Two loads can have the same total force and create different effects. In the 4 m example, 12 kN/m over the entire span totals 48 kN. For a separate idealised model, a single 48 kN force at midspan gives the same 24 kN reaction at each support, but the maximum moment becomes PL/4 = 48 kNm, twice the uniform-load value of 24 kNm.
This is a comparison of elementary static models, not a new input option in our calculator. The online tool represents a full-span uniform load and cannot model that point load. Dividing a column reaction by the span does not make it structurally equivalent to a UDL.
Test the effect of the measured span
With the same section and total 12 kN/m uniform load, changing the support-centre distance from 4.0 m to 4.2 m changes these outputs:
| Model span | Reaction at each support | Maximum moment | Calculated deflection |
|---|---|---|---|
| 4.0 m | 24.0 kN | 24.00 kNm | 8.14 mm |
| 4.2 m | 25.2 kN | 26.46 kNm | 9.89 mm |
Deflection increases by about 21.6% because the formula contains the fourth power of span. The comparison is useful for checking input errors; it does not establish a permissible measurement tolerance. The engineer determines the design span and confirms the site dimensions needed for fabrication.
Keep a record that can be checked
Save the complete section designation, the source of the loads, whether self-weight is included, the span definition and the calculator inputs. A screenshot showing only “within target” loses the assumptions that make the result meaningful. Include any point loads or support questions separately, even though this tool cannot calculate them.
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.