Quick reference
| Size | Pitch (mm) | Stress area (mm²) | Across flats (mm) | Normal hole (mm) |
|---|---|---|---|---|
| M8 | 1.25 | 36.6 | 13 | Not in table |
| M10 | 1.5 | 58.0 | 16 | Not in table |
| M12 | 1.75 | 84.3 | 18 | 13 |
| M16 | 2 | 157 | 24 | 18 |
| M20 | 2.5 | 245 | 30 | 22 |
| M24 | 3 | 353 | 36 | 26 |
| M27 | 3 | 459 | 41 | 30 |
| M30 | 3.5 | 561 | 46 | 33 |
Key points
- Property class: first number × 100 = tensile strength; both multiplied × 10 = yield (N/mm²).
- 8.8 and 10.9 are the usual structural classes; 4.6 is lower strength.
- Normal holes (BS EN 1090-2): M12–M14 +1mm, M16–M24 +2mm, M27 and up +3mm.
- Torque depends on friction: see the bolt and fixing guide for indicative maximums, and use the project specification for structural bolts.
Terms explained in the A–Z glossary.
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Two-page summary of the figures on this page, with sources. Free to share with credit to Koyda Ltd.
Property classes (ISO 898-1)
| Class | Tensile Rm min (N/mm²) | Yield min (N/mm²) | Eurocode fyb / fub (N/mm²) |
|---|---|---|---|
| 4.6 | 400 | 240 (ReL) | 240 / 400 |
| 8.8 (d ≤ 16mm) | 800 | 640 (Rp0.2) | 640 / 800 |
| 8.8 (d > 16mm) | 830 | 660 (Rp0.2) | 640 / 800 |
| 10.9 | 1040 | 940 (Rp0.2) | 900 / 1000 |
Design to Eurocode 3 uses the nominal values from BS EN 1993-1-8 Table 3.1 in the last column. Nuts should be the matching class (8 for 8.8, 10 for 10.9).
Clearance holes (BS EN 1090-2)
- Normal round holes: M12 and M14 +1mm; M16 to M24 +2mm; M27 and larger +3mm.
- Hole tolerance ±0.5mm unless the execution specification states otherwise.
- Oversize and slotted holes have their own clearances and reduce slip resistance. They must be specified by the designer.
- The table does not give clearances for M8 and M10. Use the execution specification.
Tightening torques
There is no single torque value that holds across UK structural work. Torque depends on the friction of the thread and nut face, which changes with coating (black, galvanised, zinc plated) and lubrication. For structural bolts, use the project specification: non-preloaded bolts are snug tight (BS EN 1090-2, 8.3). For preloaded assemblies, follow BS EN 1090-2 and BS EN 14399 with the manufacturer's k-class data.
For general engineering, the bolt and fixing guide gives indicative maximum torques and preloads at a friction you choose, from the VDI 2230 table published by Bossard, with ISO 273 clearance holes and the BS EN 1993-1-8 edge and pitch distances.
Frequently asked questions
What does 8.8 mean on a bolt?
It is the property class to ISO 898-1. The first 8 means a nominal tensile strength of 800 N/mm². The second 8 means the yield is 80% of that, 640 N/mm². Above M16 the minimum tensile strength is 830 N/mm² and yield 660 N/mm².
What size hole for an M20 bolt?
22mm for a normal round hole in structural steelwork to BS EN 1090-2: the bolt diameter plus 2mm. The usual tolerance is ±0.5mm unless the execution specification says otherwise.
What torque should I use for an M16 8.8 bolt?
Use the value in your project specification. Non-preloaded structural bolts are usually tightened snug tight, and preloaded bolts follow BS EN 1090-2 with the bolt maker's data. For general engineering, the Koyda bolt and fixing guide gives the indicative maximum torques from the VDI 2230 table published by Bossard: for M16 8.8 that is 180 Nm at a friction of 0.10 and 230 Nm at 0.14. The right figure depends on friction, lubrication and coating, so a single value can be badly wrong.
Can I use a 4.6 bolt instead of 8.8?
Not without the engineer's agreement. A 4.6 bolt has half the tensile strength of an 8.8 bolt. Use the class shown on the drawings and match nuts and washers to it.
What spanner fits an M12 bolt?
18mm for ISO 4014 bolts and ISO 4032 nuts. Older DIN 931 and 934 parts used 19mm for M12, so check which you have.
Sources
- ISO 898-1:2013 mechanical properties of fasteners: bolts, screws and studs, Table 3 (ISO)
- BS EN 1993-1-8 Table 3.1: nominal yield and ultimate strengths of bolts (BSI)
- BS EN 1090-2:2018+A1:2024 execution of steel structures, 6.6 holing: nominal clearances and ±0.5mm tolerance (summarised in SCI ED018)
- SteelConstruction.info GN 5.08: hole sizes and positions for preloaded bolts
- ISO 4014 / ISO 4032 hexagon bolts and nuts: widths across flats; ISO 261/262 coarse thread pitches (ISO)
- Bossard, Preload and tightening torques (01-2025): VDI 2230 table, metric coarse threads
General guidance only, not design advice for a specific project. Building regulations guidance changes: check the current edition of each document, and get project-specific design from a competent engineer. Koyda Ltd accepts no liability for decisions made using this page.