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Beam load and steel size calculator
For early planning of a knock-through or extension. Estimate the load on a simply supported beam, its bending moment and shear, and the minimum steel section properties. An engineer must still design and sign off the beam.
- Design load (ultimate)
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- Characteristic loads
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- Bending moment
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- Shear / end reaction
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- Minimum Wpl,y
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- Minimum Iy (deflection)
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- Your section
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Not a structural design. Ignores lateral-torsional buckling, bearing, padstones, connections and point loads. Building Control will need calculations from a structural engineer.
Read this first. A beam that carries a wall, floor or roof is structural work. Building Regulations need a design by a competent person, normally a structural engineer, and Building Control will ask for their calculations. Use this calculator to understand the loads, have a sensible conversation with your engineer, and budget for the steel. Do not order or install a beam from these results alone.
What the calculator does
It treats the beam as simply supported (resting on a support at each end, free to rotate) with a uniformly distributed load along its length. That is the most common case for a beam under a floor in a house.
- Loads. The floor helper turns area loads (kN/m²) into line loads (kN/m) on the beam. A beam carrying joists from one side takes half the joist span; from both sides it takes two halves. Add walls, roof loads and the beam's own weight as extra loads.
- Ultimate load. Characteristic loads are factored using BS EN 1990: 1.35 × dead + 1.5 × imposed.
- Bending and shear. For a simply supported UDL, maximum bending moment is wL²/8 and the end reaction is wL/2.
- Bending strength. The minimum plastic section modulus is M ÷ fy (with the UK partial factor γM0 = 1.0), for S275 or S355 steel.
- Stiffness. The minimum second moment of area so deflection stays within span/360 under imposed load and span/250 under total load, using E = 210,000 N/mm².
Worked example
A 4 m beam carrying a domestic timber floor with joists spanning 4 m from one side: the floor load is 2 m × 0.5 = 1 kN/m dead and 2 m × 1.5 = 3 kN/m imposed. Add 9 kN/m for a block wall above and the beam's weight, and 2 kN/m of roof imposed load: Gk = 10, Qk = 5 kN/m.
- Ultimate load 1.35 × 10 + 1.5 × 5 = 21 kN/m
- Bending moment 21 × 4² ÷ 8 = 42 kNm; end reactions 42 kN
- Minimum Wpl,y in S275: 153 cm³
- Minimum Iy: 1,488 cm⁴ (total load deflection governs)
Your engineer would then choose a section with more than both values, and check things this calculator leaves out.
What this calculator does not check
- Lateral-torsional buckling. An unrestrained steel beam can twist sideways long before it reaches its bending strength. This often governs domestic steels.
- Point loads from posts, other beams or chimney breasts.
- Bearings and padstones: the wall under each end has to carry the end reaction without crushing.
- Fire protection, connections, temporary propping and the wall or foundation below.
Questions
Do I need a structural engineer for a steel beam?
Yes, in practice. Removing a load-bearing wall is building work under the Building Regulations, and Building Control will ask for structural calculations from a competent person.
What is the imposed load for a house floor?
1.5 kN/m² for domestic floors under the UK National Annex to BS EN 1991-1-1, plus an allowance for lightweight partitions where they apply.
Where do I find Wpl and Iy for a steel section?
In published section property tables for UK universal beams and columns, or from your steel supplier. Engineers use the same values.
Why does deflection often govern?
Long spans can be strong enough but still too bouncy. Limiting deflection to span/360 under imposed load keeps ceilings from cracking and floors from feeling springy.
Results are estimates for planning and ordering. Check quantities with your supplier and product data sheets, and anything structural or regulated with a qualified professional. See our terms.