If you are planning a renovation, knocking through a wall, or converting a loft, you will almost certainly need a steel beam. But steel stockyard catalogues look like alphabet soup: 127×76×13, 203×133×25, 254×146×31. What do these numbers mean? And which one do you need?
This guide explains RSJ beam sizes in plain English. We will break down the numbering system, walk through the most common UK sizes, and give you a quick-reference chart.
How to Read RSJ Beam Sizes (Depth × Width × Mass per metre)
Every RSJ beam size is written as three numbers:
Depth (mm) × Width (mm) × Mass per metre (kg/m)
Take 203×133×25, the most common beam in British houses:
- 203 = depth in millimetres (vertical height)
- 133 = flange width in millimetres (horizontal top and bottom plates)
- 25 = mass in kilograms per metre of length
A 4-metre 203×133×25 weighs roughly 100 kg; a 305×305×118 of the same length weighs 472 kg. That third number dictates lifting requirements and whether your walls can carry the load.
These dimensions are standardised under BS EN 10365 (BSI Group). UK engineers also use the “blue book” from the Institution of Structural Engineers and the Steel Construction Institute.
Note: “RSJ” is often used colloquially for any steel beam, but technically it refers to a Rolled Steel Joist with tapered flanges. Most modern homes use UBs (Universal Beams) with parallel flanges, which are more efficient. Builders and homeowners use the terms interchangeably, and we will do the same here.
Small RSJ Sizes: 127×76×13 to 178×102×19 (door lintels, small windows)
Small beams in the 127 mm to 178 mm depth range are ideal for minor structural work. They are light enough for two people to handle and fit neatly into cavity walls.
127×76×13
At 13 kg/m, the smallest standard universal beam. Ideal for:
- Single door lintels in non-load-bearing partitions
- Small window openings up to 1.5 m
- Light garage door openings
152×89×16
A small step up with more stiffness. Typical uses:
- Wider doorways up to 1.8 m
- Small windows under modest roof loads
- Light first-floor joist runs over narrow openings
178×102×19
Where small beams start to feel genuinely structural. Common applications:
- Patio door openings up to 2 m
- Larger windows under a pitched roof
- Light internal knock-throughs supporting one floor
These beams are rarely used for major load-bearing walls, but they are perfect for outbuildings and light renovation work.
Medium RSJ Sizes: 203×102×23 to 203×203×52 (the 203×133×25 workhorse)
This is the range where most domestic steel beams fall. If you are removing a wall in a typical British house, your engineer will likely specify something here.
203×102×23
A light beam with a narrow flange. Often used for:
- Short spans (up to 2.5 m) carrying floor joists
- Single-storey extensions with minimal roof load
- Supporting a single floor over a small opening
203×133×25
This is the most popular RSJ size for residential projects in the UK. It strikes a near-perfect balance between strength, weight, and cost. You will see it specified for:
- Standard internal wall removals (2.5 m to 3.5 m openings)
- Supporting first-floor joists and a modest roof load
- Kitchen-diner knock-throughs in terraced and semi-detached homes
For a deep dive, read our dedicated 203×133×25 RSJ beam guide.
203×133×30
When loads are higher—supporting a bedroom floor, bathroom, and roof—the engineer may bump up to the 30 kg/m version. The extra 5 kg/m gives a meaningful increase in bending resistance.
203×203×46 and 203×203×52
Deeper, squarer beams with extra flange width for lateral stiffness. Typical uses:
- Supporting two floors over a wide opening
- Commercial refurbishment with higher imposed loads
- Beams that also act as stability elements
For most homeowners, the 203×133×25 or 203×133×30 will cover the majority of internal wall removals.
Large RSJ Sizes: 254×102×22 to 254×254×89 (lofts, wide knockthroughs)
These beams are commonly used for larger open-plan spaces, loft conversions, and openings where two or more floors are supported.
254×102×22
Surprisingly, this beam is lighter than some 203 mm beams because it is deeper but much narrower. The extra depth gives better span capability, but the narrow flange limits load capacity before it becomes unstable. Engineers use it for:
- Long, lightly loaded spans up to 4 m
- Roof ridge beams
- Loft conversions where the beam only supports the roof and a small floor area
For more on loft conversions, see our guide to RSJ for loft conversion.
254×146×31 and 254×146×37
The large-beam equivalents of the popular 203×133×25. They offer significantly more capacity and are frequently specified for:
- Wide kitchen-diner openings (3.5 m to 4.5 m)
- Supporting two floors over a generous span
- Bungalow renovations with substantial roof loads
254×254×73 and 254×254×89
These beams can weigh over 300 kg for a 4-metre length. They are used for:
- Large structural openings in commercial or heavy domestic work
- Supporting multiple floors and a roof over a wide bay
- Projects where the beam provides lateral stability to the building
At this size, you will need a steel trolley, possibly a mini crane, and several strong pairs of hands.
Extra Large RSJ Sizes: 305×102×25 to 305×305×118 (commercial, long spans)
These beams are overkill for most standard houses, but essential when the span is long or the loads are high.
305×102×25 and 305×102×28
Specialist long-span, light-load beams. The extreme depth-to-width ratio makes them efficient for roof loads across wide bays, but they need proper restraint. Uses:
- Wide-span roof beams in industrial units
- Agricultural buildings with large clear spans
- Light commercial mezzanines
305×165×40 and 305×165×46
A balanced profile for heavier commercial work. These beams can comfortably span 5 m to 6 m with significant floor loads. Common in:
- Retail fit-outs
- Office renovations
- Multi-storey steel frames
305×305×97 and 305×305×118
Serious structural members. A 305×305×118 weighs 118 kg/m, so a 5-metre length is nearly 600 kg. Used for:
- Primary columns and beams in steel-framed buildings
- Heavy industrial applications
- Very long spans with high loads
For most homeowners, the 305 mm range is unlikely to be relevant unless you are working on a large commercial project.
Quick-Reference RSJ Size Chart
Here is an at-a-glance chart for the most common UK universal beam sizes. For a detailed load table, see our complete RSJ size chart.
| Designation | Depth (mm) | Width (mm) | Weight (kg/m) | Typical Span | Best Use |
|---|---|---|---|---|---|
| 127×76×13 | 127 | 76 | 13 | Up to 1.5 m | Door lintels, small windows |
| 152×89×16 | 152 | 89 | 16 | Up to 1.8 m | Wider doorways, light joists |
| 178×102×19 | 178 | 102 | 19 | Up to 2.2 m | Patio doors, medium windows |
| 203×102×23 | 203 | 102 | 23 | Up to 2.8 m | Short spans, single floor |
| 203×133×25 | 203 | 133 | 25 | 2.5–3.5 m | Most popular for homes |
| 203×133×30 | 203 | 133 | 30 | Up to 3.8 m | Heavier domestic loads |
| 254×102×22 | 254 | 102 | 22 | Up to 3.5 m | Loft conversions, roof beams |
| 254×146×31 | 254 | 146 | 31 | 3.0–4.5 m | Wide knock-throughs, two floors |
| 254×146×37 | 254 | 146 | 37 | 3.5–5.0 m | Large openings, heavy roofs |
| 305×165×40 | 305 | 165 | 40 | 4.0–6.0 m | Commercial spans, retail |
| 305×305×97 | 305 | 305 | 97 | 5.0–8.0 m | Heavy commercial, primary steel |
This chart gives typical spans for guidance only. The actual beam you need depends on the exact load, support conditions, and building geometry. Always consult a structural engineer.
What Size RSJ Do I Need?
This is the question every homeowner asks. The honest answer: it depends on the load, the span, and the structural arrangement. But there are useful rules of thumb.
Rule of Thumb for Domestic Work
For a standard British house with brick cavity walls and timber floors:
- 1.5 m to 2.5 m opening: 152×89×16 or 178×102×19
- 2.5 m to 3.5 m opening: 203×133×25 or 203×133×30
- 3.5 m to 4.5 m opening: 254×146×31 or 254×146×37
- 4.5 m to 6.0 m opening: 305×165×40 or larger
These are broad guidelines. A 3.5 m opening with only a lightweight roof might manage with a 203×133×30, while the same span supporting two floors and a heavy tiled roof might need a 254×146×37 or 305×165×40.
When to Upsize
Your engineer may specify a larger beam. Common reasons:
- Heavy loads above: baths, kitchens, tiled roofs, or masonry walls
- Point loads: chimney breasts or internal walls landing on the beam
- Longer spans: deflection often governs, not just strength
- Limited end bearings: thin walls need a wider flange to spread the load
- Future-proofing: preparing for a future loft conversion
For a quick estimate, try our beam size calculator. It gives a sensible starting point, but it is not a substitute for professional design.
European IPE Sections vs UK RSJ Sizes
You might come across “IPE” beams when comparing quotes. These are the European equivalent of UK universal beams with a different naming system.
- IPE 140 ≈ 140 mm deep, similar to a UK 152×89×16
- IPE 200 ≈ 200 mm deep, similar to a UK 203×133×25
- IPE 240 ≈ 240 mm deep, similar to a UK 254×146×31
- IPE 300 ≈ 300 mm deep, similar to a UK 305×165×40
IPE sections have slightly different flange widths and weights, but they are structurally similar. Most UK stockholders carry both, and engineers design with either provided they meet British and European standards.
One difference: IPE beams sometimes have slightly rounded flange edges, which can affect how neatly they sit on padstones. Worth checking before ordering if appearance matters.
FAQs
What is the most popular RSJ size for a house?
The 203×133×25 is by far the most commonly specified beam for residential projects in the UK. It is deep enough to span typical internal wall removals (2.5 m to 3.5 m), light enough to handle on site without a crane, and competitively priced. For more detail, see our dedicated 203×133×25 guide.
Can I use a smaller beam to save money?
Tempting, but risky. Undersizing a beam can lead to excessive sagging, cracking in walls and ceilings, and in extreme cases, structural failure. The cost difference between a 203×133×25 and a 203×133×30 is usually only £30 to £50 for a 4-metre length. Saving that small amount is not worth compromising the safety of your home. Always follow your structural engineer’s specification.
Does beam depth or width matter more?
Depth matters more for span and stiffness. A deeper beam can span further and deflect less under load. However, width matters for stability and bearing. A wider flange spreads the load onto the supporting wall and resists twisting (lateral torsional buckling). That is why engineers choose beams with a balanced depth-to-width ratio, not just the deepest beam available.
How far can a 203×133×25 span?
In typical domestic conditions, a 203×133×25 can span roughly 3.0 m to 3.5 m while supporting a single floor and a modest roof load. If the load is very light—say, a non-load-bearing partition wall only—it might stretch to 4.0 m. If the load is heavy, or if it supports two floors, the safe span may drop to 2.5 m or less. Always get a structural calculation for your specific situation.
What size RSJ for a 4m opening?
For a 4-metre opening in a typical house, you will usually need a 254×146×31 or 254×146×37, depending on the load. If the beam only supports a lightweight roof, a 254×102×22 might suffice. If it supports two floors and a heavy roof, you may need a 305×165×40. Again, this is a general guide—your structural engineer will calculate the exact size.
Important Disclaimer
The information in this article is provided for general guidance only. Every building is different, and the correct steel beam size depends on a detailed structural analysis of your specific project, including load calculations, support conditions, and compliance with current Building Regulations.
You must appoint a chartered structural engineer to design and specify any load-bearing steelwork. All structural alterations must also be notified to Building Control and approved before work begins. RSJ.info and the authors accept no liability for any loss, damage, or injury arising from the use of the information provided here. When in doubt, consult a professional.
