Lag screw pilot hole chart
Pilot-hole sizes for every common lag bolt, ¼" through ¾", across softwood, medium and hardwood. Fractions and millimetres.
| Lag size | Softwood | Medium | Hardwood |
|---|---|---|---|
| 1/4" | 3/32"2.4 mm | 5/32"4 mm | 3/16"4.8 mm |
| 5/16" | 9/64"3.6 mm | 3/16"4.8 mm | 13/64"5.2 mm |
| 3/8" | 11/64"4.4 mm | 15/64"6 mm | 1/4"6.4 mm |
| 1/2" | 15/64"6 mm | 5/16"7.9 mm | 11/32"8.7 mm |
| 5/8" | 5/16"7.9 mm | 13/32"10.3 mm | 29/64"11.5 mm |
| 3/4" | 13/32"10.3 mm | 1/2"12.7 mm | 9/16"14.3 mm |
Softwood — pine, spruce, cedar, fir · Medium — Douglas fir, southern yellow pine · Hardwood — oak, maple, birch.
The sizes above are the pilot for the threaded portion. Also drill a clearance hole the size of the bolt’s nominal diameter (a ⅜" lag → a ⅜" hole) through the top piece being fastened.
How to read it
Each row is the pilot-hole size for that lag diameter, in the softwood, medium or hardwood column. Remember a lag needs two holes: the pilot above, plus a clearance hole matching the bolt’s nominal diameter through the top piece, so the lag draws the joint tight.
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Where these numbers come from
Pilot sizes for lag bolts vary by source and wood type — these follow common split-safe guidance cross-referenced across fastener references (Monster Bolts and a second agreeing chart). They lean toward not splitting the wood, which is the usual DIY concern, rather than squeezing out maximum pull-out strength.
For major structural or load-bearing work — deck ledgers, heavy timber connections, anything holding significant weight — follow the lag manufacturer’s specifications or an engineer’s guidance. These general sizes are for typical DIY use.