Estimate steel beam size for domestic openings and load-bearing walls
This is an estimate only. All structural steelwork MUST be designed by a qualified structural engineer and comply with Building Regulations Approved Document A. Do not use this calculator as a substitute for professional structural design.
When removing a load-bearing wall, creating an open-plan living space, or installing a new opening for a door or window, a steel beam is usually required to carry the loads above. Choosing the right beam size is a structural engineering decision that must comply with Building Regulations Approved Document A and be verified by a qualified structural engineer. However, having a rough idea of the likely beam size before you speak to an engineer or builder helps you plan the project, estimate costs, and understand whether your proposed opening is practically achievable. This calculator provides a simplified estimate of steel beam size for common UK domestic scenarios. It covers universal beams (UBs) for floor and roof loads, and standard proprietary lintels for door and window openings. The results are based on typical domestic loading conditions and should be treated as indicative only. Every structural situation is unique, and factors such as point loads, asymmetric loading, existing structural conditions, and bearing capacity all affect the final beam specification. A structural engineer's calculation is not optional -- it is a legal requirement under the Building Regulations.
To estimate a beam size for your project: 1. Enter the clear span in metres. This is the distance between the inner faces of the two supporting walls or columns. For a typical knocked-through reception room, this might be 3-4 metres. For a wider opening spanning across the back of a house, it could be 5-6 metres or more. 2. Select the load type. "Single Floor" means the beam supports one floor above (the most common domestic scenario when removing a ground-floor wall in a two-storey house). "Double Floor" means two floors above (e.g. removing a wall at ground level in a three-storey house). "Roof Only" means the beam supports only the roof structure with no floors above (e.g. creating an opening in an upstairs wall of a bungalow). "Lintel" is for door and window openings in masonry walls, using standard proprietary lintels. 3. Optionally adjust the bearing length. This is the minimum length of beam that must rest on solid support at each end. The standard minimum is 150mm, and this is built into the cost of the padstones (concrete blocks placed under each end of the beam). 4. Review the suggested beam size and its approximate weight per metre. Use this information when discussing your project with a structural engineer or builder. Remember that the final specification may differ based on site-specific factors that this simple estimator cannot account for.
This calculator uses a simplified lookup table approach rather than full structural calculations. In reality, beam sizing involves calculating applied loads (dead loads from floor, wall, and roof constructions plus imposed loads from occupancy and furniture), determining bending moments and shear forces, then selecting a steel section with adequate moment capacity, shear capacity, and acceptable deflection. The lookup table is based on typical UK domestic conditions: floor dead load approximately 1.5 kN/m2, floor imposed load 1.5 kN/m2 (per BS EN 1991-1-1 UK National Annex Category A), roof dead load approximately 1.0 kN/m2, and a load width (tributary area) of approximately 2-3 metres per side of the beam. For single floor loads, the suggested beam sizes are: up to 2m span use 152x89 UB (16 kg/m), 2-3m span use 178x102 UB (19 kg/m), 3-4m span use 203x133 UB (25 kg/m), 4-5m span use 254x146 UB (31 kg/m), and 5-6m span use 305x165 UB (40 kg/m). For double floor loading, the calculator steps up one beam size from the single floor recommendation to account for the additional load. For roof-only loading, it steps down one size since the load is lighter. Lintel loads use standard proprietary lintel sizes (such as Catnic or IG Lintels) which are designed for specific span ranges and are available from builders' merchants. All beam sizes reference standard UK universal beam sections to BS 4 / BS EN 10365, which are the sections most commonly stocked by UK steel suppliers. The beam weight in kg/m is useful for estimating the total weight for handling and delivery purposes -- a 305x165x40 UB spanning 5m weighs approximately 200kg and requires mechanical lifting.
Important: this calculator is for guidance only and must not be used as a substitute for professional structural engineering design. Under UK Building Regulations, structural alterations to load-bearing elements require Building Control approval, and the structural design must be carried out or verified by a competent person. A structural engineer will produce calculations to Eurocode 3 (BS EN 1993) that consider your specific loading conditions, connection details, fire protection requirements, and lateral restraint. They will also specify padstone sizes, temporary propping requirements during installation, and any fire protection (typically intumescent paint or boarding) needed to achieve the required fire resistance period. Budget between 300 and 800 pounds for a structural engineer's calculation and drawing for a typical domestic beam, which is a small fraction of the overall project cost and provides the legal documentation needed for Building Control sign-off.