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5/16-inch bolt size
1/2 in wrench
across-flats spanner / socket size for 5/16 in
5/16-inch bolt — trailers, machinery and brackets.
Full specification
| System | Imperial UNC |
| Nominal diameter | 5/16 in (7.94 mm) |
| Thread pitch | 18 TPI |
| Wrench / spanner (across flats) | 1/2 in |
| Tap drill | F (6.5 mm) |
| Clearance hole | 21/64 in |
| Common grade / class | Grade 5 |
| Typical torque | 31 Nm (23 ft·lb) |
| Closest other-system size | ~M8 |
🔧 Torque unit converter
Pre-filled with the typical torque for 5/16 in (Grade 5, dry). Edit any field to convert.
⚠️ Torque values here are general references for Grade 5 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 5/16 in
5/16-18 is common on trailers, mounts and medium brackets.
5/16 in is the closest imperial size to M8 but is not interchangeable.
Frequently asked questions
- What size wrench or spanner fits 5/16 in?
- 5/16-inch bolt uses a 1/2 in wrench or spanner, measured across the flats of the hex head.
- What is the torque for 5/16 in?
- A typical tightening torque for 5/16-inch bolt in Grade 5 on dry threads is about 31 Nm (23 ft·lb). Always confirm against the manufacturer spec for important joints.
- What thread pitch and tap drill does 5/16 in use?
- 5/16-inch bolt has a thread pitch of 18 TPI and uses a F (6.5 mm) tap drill, with a 21/64 in clearance hole.