Steel Beam RSJ Calculator — UDL, Moment & Beam Selection

First calculate the total UDL (uniformly distributed load) from the floor area loading multiplied by the supported width. Then calculate the bending moment using M = wL²/8 (for simply supported beams with UDL). Finally, select a beam with a moment capacity greater than the calculated bending moment from the steel section tables. This calculator uses S275 grade steel.

M = wL²/8 is the formula for the maximum bending moment of a simply supported beam with a uniformly distributed load. M is the moment in kNm, w is the load in kN/m, and L is the span in metres. This is the most common loading condition for domestic beams supporting floors or roofs above. Point loads require different calculations.

Yes, Building Regulations require structural calculations by a competent person (usually a chartered structural engineer) for all structural alterations including steel beams. This calculator provides indicative sizing only — it does not account for lateral torsional buckling, shear, deflection limits, web bearing, or connection design. Always get professional calculations. See our Steel Beam Calculator for an alternative sizing estimate.

For domestic floors (BS EN 1991-1-1, Category A): imposed load = 1.5 kN/m² and dead load typically 1.0-2.0 kN/m² depending on floor construction. These are factored by partial safety factors (1.35 for dead, 1.5 for imposed) for ultimate limit state design. Roofs use a lower imposed load — 0.6 kN/m² for a Category H roof with maintenance access only (UK NA to BS EN 1991-1-1), reducing toward zero as the roof slope steepens; add snow load per BS EN 1991-1-3 where it governs (upland/northern sites). Always verify loads with your structural engineer.

Steel price is supplier- and quantity-dependent (2026 estimate, not a fixed rate). Buying a single short domestic beam from a builders’ merchant — often cut to length — typically works out around £2.00-3.50 per kg, which is the default in this calculator. Bulk supply-only universal-beam (UB) pricing is much lower, roughly £820-1,080 per tonne (≈ £0.82-1.08 per kg) for stock sections. A common 203x133x25 UB at 4m span weighs about 100 kg, so material cost ranges from roughly £85-110 (bulk) to £250-350 (single merchant cut). Add £500-2,000+ for installation including padstones, temporary support and making good. Delivery and crane costs are additional. Edit the price field to match your own quote.