S235, S275 and S355 are the three structural carbon steel grades used across nearly all UK fabrication, all made to BS EN 10025-2. The number after the S is the steel’s minimum yield strength in N/mm² (also written MPa): 235, 275 and 355 for material up to 16 mm thick. S275 is the everyday default for general steelwork, S355 is the higher-strength choice for long spans and heavy loads, and S235 covers lighter, less critical fabrication. The chart below sets out the full comparison; the sections beneath explain what each column means and how the S275 vs S355 grades pair, which most engineers choose between, fits into the wider picture.
| Grade | Min yield ≤16 mm | Min yield >16-40 mm | Tensile range | Typical impact sub-grades | Typical uses | Relative cost |
|---|---|---|---|---|---|---|
| S235 | 235 N/mm² | 225 N/mm² | 360-510 N/mm² | JR (most common) | Light and non-critical fabrication, general engineering, secondary members | Lowest |
| S275 | 275 N/mm² | 265 N/mm² | 410-560 N/mm² | JR (most common) | UK everyday structural default: extensions, small commercial buildings, lintels, brackets | Standard |
| S355 | 355 N/mm² | 345 N/mm² | 470-630 N/mm² | J0, J2 (most common), K2 for demanding cases | Long spans, heavy loads, portal frames, bridges, weight-sensitive frames | Higher |
Why the numbers differ
Yield strength is the point at which steel stops springing back and starts to deform permanently under load. The higher the number, the more stress the section can carry before that happens. Moving from S235 to S275 to S355 is roughly a 17 per cent step, then a further 29 per cent step, in that yield strength, which is why the same beam in a higher grade can often be swapped for a smaller or lighter section carrying the same load. Note that the minimum yield drops slightly as material gets thicker; that is a property of rolled steel generally, not something specific to one grade, so the chart shows both the ≤16 mm and the 16-40 mm figures. At 40-63 mm thickness the figures fall again, to 215, 255 and 335 N/mm² for S235, S275 and S355 respectively.
Tensile strength, the second column that matters, is the stress at which the steel actually fractures rather than just yielding. It gives a working range (for example 410-560 N/mm² for S275 across 3-100 mm thickness) rather than a single figure, and it always sits well above the yield strength, which is the margin an engineer’s calculation relies on.
S235: where it fits
S235 is the lightest of the three grades and the least commonly specified on modern UK drawings, but it still turns up on secondary members, general engineering fabrication and non-critical brackets and supports where the extra strength of S275 is not needed. It is cheaper per tonne than S275, though the saving is usually small once fabrication and handling are counted, so many fabricators stock S275 as their general-purpose default instead and reserve S235 for jobs that specifically call for it.
S275: the UK default
S275 is the grade you will see most often on everyday UK structural steelwork: beams and columns in house extensions, lintels over openings, brackets, posts and frames where loads are moderate and section size is not tightly constrained. It is widely stocked, economical, and more than adequate for the bulk of the structural steelwork TC Rowan fabricates. For a deeper look at where S275 earns its place against the stronger grade, see our S275 steel guide.
S355: strength where it counts
S355 is the grade to reach for when strength-to-weight matters: long spans, heavily loaded beams and columns, portal frames, bridges and larger commercial or industrial structures where a lighter, slenderer section pays for itself in reduced steel tonnage and easier handling. The trade-off is a higher price per tonne, so an engineer will often mix grades on one project, using S355 only where the load genuinely demands it and S275 for the rest of the frame. Our S355 steel guide covers its uses in more depth, and if your specification names a particular impact sub-grade, the S355JR vs S355J0 vs S355J2 comparison explains exactly what changes between them.
Stiffness and weight do not change with grade
It is a common assumption that a stronger grade also means a stiffer or lighter section, but that is not how the numbers work. S235, S275 and S355 share the same Young’s modulus, around 210,000 N/mm², and the same density, 7,850 kg/m³, so stiffness per section size and weight per metre are identical across all three grades. Only the yield and tensile strength change. That matters for any member governed by deflection rather than strength: upgrading the grade on a deflection-critical beam saves nothing, because the steel is exactly as stiff as before.
Beyond S355
For specialist heavy structures with particularly demanding loads, fine-grain grades S420 and S460 exist under BS EN 10025-3 and -4, though they are rare outside major civil or industrial projects and sit well beyond the scope of typical commercial and residential steelwork.
Grade is one input, not the whole spec
A drawing rarely stops at the grade. It usually also names the impact sub-grade, the section size (see our universal beam sizes guide), and how the steel was formed, which is a separate question covered in our hot rolled vs cold rolled steel guide. For a completely different comparison, wear-resistant plate such as Hardox is not a structural grade at all; our Hardox vs S355 guide explains when you actually need one over the other. Deciding the grade for a specific job is always the structural engineer’s decision, made as part of the design calculation; TC Rowan’s role is to fabricate to that specification, working to BS EN 1090 execution standards with material traceability available on request from our own workshop at Unit 2 Thorpe Way Industrial Estate, Banbury.
For more detail on the general-purpose comparison most projects actually need, the S275 vs S355 guide is the place to start. Independent detail on yield strength and other mechanical properties is also available from the Steel Construction Institute.
Frequently asked questions
What is the main difference between S235, S275 and S355 steel?
The number is the minimum yield strength in N/mm2 under BS EN 10025-2: S235 is 235, S275 is 275 and S355 is 355 for material up to 16 mm thick. S355 is the strongest of the three and S235 the weakest, so a higher grade lets an engineer specify a smaller or lighter section for the same load. All three have identical stiffness (Young’s modulus around 210,000 N/mm2) and the same density, so the choice is about strength, not rigidity.
Is S355 always better than S275 or S235?
No. S355 costs more per tonne, so it only pays off where its extra strength is actually used, such as long spans, heavy loads or weight-sensitive frames. For light or non-critical fabrication, secondary members or a typical extension, S275 (or occasionally S235) does the job at a lower material cost. The structural engineer’s calculation decides the grade, not a default assumption that stronger is better.
What do S235JR, S275JR and S355J2 mean?
The letters after the grade describe impact toughness: how well the steel resists brittle fracture at low temperature. JR is tested at +20 degrees C, J0 at 0 degrees C, J2 at minus 20 degrees C and K2 (a tougher class, common on S355) also at minus 20 degrees C but to a higher energy absorption. S235JR, S275JR and S355J0 or S355J2 are the specifications you will see most often on UK drawings.
Does a higher steel grade reduce deflection?
No. Deflection depends on stiffness, and stiffness (Young’s modulus, around 210,000 N/mm2) is the same across S235, S275 and S355. Moving to a higher grade increases the load or stress the steel can carry before yielding, but it does nothing for a member that is governed by deflection rather than strength. That distinction is worth raising with your structural engineer if weight saving is the goal.
Not sure which grade your drawings call for, or fabricating from a mixed-grade specification? Send us the engineer’s drawings and we will supply and fabricate to the exact spec, from our own steel fabrication workshop through to erection. Get in touch for a free quote.