Deflection measures movement under load. It is different from a strength or stability check, and a small predicted deflection does not establish safety.
Use the current tool
The load calculator converts entered area loads and tributary width to a line load and adds beam self-weight. The deflection calculator instead expects a total line load with self-weight already included. Avoid counting it twice. The span comparison uses your exact span rather than choosing a nearby preset row.
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.
Reproduce the calculation
For a simply supported beam carrying a uniform service load over the full span:
- Support reaction: R = wL / 2.
- Maximum bending moment: M = wL² / 8.
- Elastic deflection: δ = 5wL⁴ / (384EI).
Use kN/m and metres for reaction and moment. For deflection use N/mm, mm and mm⁴: 1 kN/m equals 1 N/mm, and 1 cm⁴ equals 10,000 mm⁴. E is 210,000 N/mm², from SteelConstruction.info material properties. See SCI P183 for the uniform-load deflection expression.
At a 4 m support-centre span with a total service load of 12 kN/m, a 203×133×25 section (Iₓ = 2340 cm⁴) gives 24 kN at each support, 24 kNm maximum moment and approximately 8.14 mm deflection. L/360 would be 11.11 mm. These are arithmetic outputs for an illustrative model, not proof of suitability. In particular, an L/360 target is not a universal Building Regulations limit.
Questions to ask about a load table
- Does the span mean clear opening or distance between support centres?
- Is the load a service load or a factored design load?
- Are beam self-weight and any point loads included?
- What lateral restraint and steel grade are assumed?
- Is the stated limit for total deflection or a particular load component?
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.