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M18 bolt size
27 mm wrench
across-flats spanner / socket size for M18
M18 bolt — heavy structural connections.
Full specification
| System | Metric coarse |
| Nominal diameter | 18 mm |
| Thread pitch | 2.5 mm |
| Wrench / spanner (across flats) | 27 mm |
| Tap drill | 15.5 mm |
| Clearance hole | 20 mm |
| Common grade / class | 8.8 |
| Typical torque | 290 Nm (214 ft·lb) |
| Closest other-system size | ~11/16 inch |
🔧 Torque unit converter
Pre-filled with the typical torque for M18 (8.8, dry). Edit any field to convert.
⚠️ Torque values here are general references for 8.8 on clean, dry threads. Lubrication, coatings, and the joint material change the figure significantly. For structural, automotive, or safety-critical work, always use the torque specified by the manufacturer or engineer.
About M18
M18 sits between the more common M16 and M20 in heavy structural work.
M18 is relatively rare — confirm availability before designing it into a build.
Frequently asked questions
- What size wrench or spanner fits M18?
- M18 bolt uses a 27 mm wrench or spanner, measured across the flats of the hex head.
- What is the torque for M18?
- A typical tightening torque for M18 bolt in 8.8 on dry threads is about 290 Nm (214 ft·lb). Always confirm against the manufacturer spec for important joints.
- What thread pitch and tap drill does M18 use?
- M18 bolt has a thread pitch of 2.5 mm and uses a 15.5 mm tap drill, with a 20 mm clearance hole.