S275 vs S355 Steel: Which Structural Grade Do You Need?
S275 and S355 are the two structural steel grades you'll meet most often in the UK, and the difference comes down to strength. S355 is roughly 29% stronger than S275, which lets you use a smaller, lighter section for the same load — but S275 is perfectly adequate, and often more economical, for plenty of jobs. This guide compares S275 vs S355 on yield strength, weldability, cost and typical uses so you can specify the right grade with confidence.
S275 vs S355: The Quick Answer
Choose S355 when you want maximum strength for the lightest section — it is the modern default for structural beams and columns, long spans and heavily loaded members. Choose S275 for general fabrication, plate, sheet, bar and lightly loaded work where its lower strength is more than enough and it can be the cheaper option. Both are weldable, low-carbon structural steels to the same standard — the choice is driven by load, span and cost, not by quality.
What Do S275 and S355 Mean?
Both grades are defined by BS EN 10025-2, the European standard for hot-rolled structural steel. The "S" stands for structural steel, and the number is the minimum yield strength in N/mm² (MPa) for the thinnest material: S275 has a 275 N/mm² yield, S355 has 355 N/mm². Yield strength is the stress at which steel starts to deform permanently — the higher the number, the more load the same section can carry before it yields.
Yield Strength Compared
S355's 355 N/mm² yield is about 29% higher than S275's 275 N/mm², so for the same loading you can often drop to a smaller, lighter S355 section. Note that yield strength reduces as material gets thicker, for both grades:
| Nominal thickness | S275 yield (N/mm²) | S355 yield (N/mm²) |
|---|---|---|
| ≤ 16 mm | 275 | 355 |
| > 16 ≤ 40 mm | 265 | 345 |
| > 40 ≤ 63 mm | 255 | 335 |
| > 63 ≤ 80 mm | 245 | 325 |
Tensile strength follows the same pattern — roughly 410–560 N/mm² for S275 and 470–630 N/mm² for S355 in common thicknesses.
What Are the J Suffixes? (JR, J0, J2)
You'll see grades written as S275JR or S355J0. The letter-number suffix is the impact toughness rating — how well the steel resists brittle fracture when struck (the Charpy V-notch test) at a given temperature: JR is tested at +20°C (room temperature), J0 at 0°C, and J2 at −20°C. For most UK structural work J0 or JR is specified; lower-temperature or more critical applications call for J2. Rackerman's steel beams, for example, are supplied in S355J0.
Weldability and Workability
Both grades are low-carbon structural steels and weld readily with standard equipment, without preheat at typical thicknesses — one reason they dominate construction and fabrication. S275 is very slightly more forgiving to weld and form because of its lower strength, but in practice both are easy to cut, drill, bend and weld. For the wider picture on how carbon content drives weldability, see our guide on mild steel vs high carbon steel.
Cost: Is S355 More Expensive?
Per tonne, S355 usually carries a small premium over S275 because of its higher strength. But that rarely tells the whole story: because an S355 member can be smaller and lighter for the same load, the total cost of a job can be the same or even lower once you account for less steel, lighter handling and easier installation. For lightly loaded or non-critical work, S275 is typically the more economical choice; for optimised structural design, S355 often wins overall.
Which Grade Should You Choose?
For structural beams and columns, long spans and heavily loaded members, S355 is the sensible modern default and is what most engineers now specify. For general fabrication, plate, sheet, bar, brackets and lightly loaded frames, S275 is usually all you need and can save money. Critically, the grade for any load-bearing element should come from a structural engineer's calculation — they will state both the section size and the steel grade for your specific design.
Buy S275 and S355 Steel at Rackerman
Rackerman supplies both grades cut to size. Structural RSJ beams and columns are stocked in S355J0, while mild steel sheet (S275), S275 round bar and the wider metal sections range cover general fabrication. See our steel grades and standards reference for full specifications.
Frequently Asked Questions
What is the difference between S275 and S355 steel?
The main difference is yield strength: S275 has a minimum yield of 275 N/mm² and S355 has 355 N/mm², making S355 about 29% stronger. Both are low-carbon structural steels to BS EN 10025-2 and weld easily. S355 allows smaller, lighter sections for the same load; S275 suits general, lightly loaded work.
Is S355 stronger than S275?
Yes. S355 has a higher yield and tensile strength than S275, so for a given section it carries more load before deforming. That lets designers use a smaller or lighter S355 member to do the same job as a larger S275 one.
What does the S in S275 mean?
The S stands for structural steel, and the number is the minimum yield strength in N/mm² for the thinnest material — 275 for S275 and 355 for S355. Both are defined by BS EN 10025-2.
What do JR, J0 and J2 mean?
They are impact-toughness ratings from the Charpy test, measuring resistance to brittle fracture at a set temperature: JR at +20°C, J0 at 0°C and J2 at −20°C. Most UK structural steel is JR or J0; J2 is used where greater low-temperature toughness is required.
Is S355 more expensive than S275?
Per tonne, S355 usually costs a little more. However, because an S355 section can be smaller and lighter for the same load, the overall job cost can be similar or lower. For lightly loaded work, S275 is often the more economical choice.
Can you weld S275 and S355 steel?
Yes, both weld readily with standard equipment and without preheat at typical thicknesses, as they are low-carbon structural steels. Follow the relevant weld procedure and any engineer's connection details for structural work.
Which grade is used for steel beams?
Modern structural beams and columns are usually specified in S355 because its higher strength allows lighter, more efficient sections. Rackerman supplies its RSJ beams in S355J0. The grade for any beam should be confirmed by a structural engineer.
Is S275 the same as mild steel?
S275 is a structural grade of mild (low-carbon) steel. "Mild steel" is the general term for low-carbon steel; S275 and S355 are specific structural grades within that family, defined and certified to BS EN 10025-2 for known strength.
Can I substitute S355 for S275?
Generally S355 exceeds S275's strength, so it can usually replace it structurally — but never change a specified grade without checking with the structural engineer, as connections, deflection and other design factors may be affected.