What size pilot hole for a lag screw?
Pick your lag bolt diameter and wood type — get the pilot hole size, plus the shank clearance hole.
Lag bolt diameter
Nominal bolt diameter — ¼" through ¾".
Wood type
Drill this pilot
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Two holes, not one
A lag needs two holes. The pilot hole above is for the threaded portion, drilled deep into the receiving piece. Then drill a clearance hole the size of the bolt’s nominal diameter (a ⅜" lag → a ⅜" hole) through the top piece being fastened, so the smooth shank slips through and the lag pulls the joint tight instead of threading into the top board.
Installing lag screws
- Center the lag in the middle third of the stud or timber — avoid end grain, which holds poorly.
- Drive with a wrench, socket, or impact, then finish by hand — over-torquing can strip the hole or snap the head.
- Add a washer under the head to spread the load.
- Outdoors, use hot-dipped galvanized or stainless lags so they don’t corrode.
Common questions
What size pilot hole for a 1/2 inch lag screw?
For a ½" lag screw, drill a 15/64" (6 mm) pilot in softwood, 5/16" (7.9 mm) in medium woods like Douglas fir, or 11/32" (8.7 mm) in hardwood. Then drill a ½" shank clearance hole through the top piece.
Do lag bolts need a pilot hole?
Yes. Lag bolts are thick enough to split wood or snap off if driven dry, so they need a pilot hole for the threaded portion, plus a clearance hole the size of the bolt's nominal diameter through the piece being fastened. Pre-drilling also lets the lag draw the joint tight instead of just boring its own way in.
Do you need a pilot hole for lag screws in softwood?
Yes, even in softwood. Softwood takes a smaller pilot than hardwood, but a lag driven without one can still split the board and is far harder to drive. Use the softwood size for the threaded pilot, plus a clearance hole matching the bolt diameter in the top piece.
Full lag screw chart
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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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.