Home › Lag & Coach Screws › 3/8 in lag
3/8-inch lag screw size
9/16 in wrench
across-flats spanner / socket size for 3/8 in lag
3/8-inch lag screw — heavy timber and ledger boards.
Full specification
| System | Imperial lag |
| Nominal diameter | 3/8 in (9.53 mm) |
| Thread pitch | coarse wood thread |
| Wrench / spanner (across flats) | 9/16 in |
| Tap drill | 1/4 in pilot |
| Clearance hole | 3/8 in |
| Common grade / class | Low-carbon steel |
| Typical torque | 47 Nm (35 ft·lb) |
| Closest other-system size | ~M10 coach |
🔧 Torque unit converter
Pre-filled with the typical torque for 3/8 in lag (Low-carbon steel, dry). Edit any field to convert.
⚠️ Torque values here are general references for Low-carbon steel 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 3/8 in lag
3/8 inch lag screws are a common choice for deck ledgers and heavy timber.
For pressure-treated wood, use hot-dip galvanised or stainless lags to resist corrosion.
Frequently asked questions
- What size wrench or spanner fits 3/8 in lag?
- 3/8-inch lag screw uses a 9/16 in wrench or spanner, measured across the flats of the hex head.
- What is the torque for 3/8 in lag?
- A typical tightening torque for 3/8-inch lag screw in Low-carbon steel on dry threads is about 47 Nm (35 ft·lb). Always confirm against the manufacturer spec for important joints.
- What thread pitch and tap drill does 3/8 in lag use?
- 3/8-inch lag screw has a thread pitch of coarse wood thread and uses a 1/4 in pilot tap drill, with a 3/8 in clearance hole.