Lag screw pilot hole chart (free printable PDF)
Pilot-hole sizes for every common lag bolt, ¼" through ¾", across softwood, medium and hardwood. Fractions and millimetres — download the free printable PDF or print it from the page.
Lag screw pilot hole chart
| 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.
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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
These are the standard published lag lead-hole sizes by wood density group — the NDS wood-group table, verified verbatim against Portland Bolt, Mudge Fasteners and Triangle Fastener’s specification sheet, which all publish the same values. Our three wood classes map to those groups: softwood = Groups III–IV, medium = Group II, hardwood = Group I. Some DIY charts run one step larger to lean split-safe; if a board is split-prone (dry, dense, near an end), stepping up one bit size is a reasonable call. For structural or code-governed work, follow the connector manufacturer’s or your code’s spec.
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.