Pilot hole size for a 1/4" lag screw (or lag bolt)
The 1/4" lag screw — also called a 1/4" lag bolt — is the lightest of the common lags: the go-to when a wood screw isn't quite enough but a big structural lag would be overkill.
Even at this size a lag needs a pilot hole. It's thick and coarse enough to split the wood or shear off if you drive it dry. Here's the right pilot for a 1/4" lag in every wood type.
Lag bolt diameter
Change the diameter to compare sizes — ¼" through ¾".
Pilot hole for a 1/4" lag, by wood type
Soft and forgiving — the smaller pilot still grips well.
Denser framing lumber — takes a mid-size pilot.
Dense and split-prone — the largest pilot, to take the pressure off.
Plus a 1/4" (6.4 mm) shank clearance hole through the piece being fastened — the smooth upper shank slips through it so the lag pulls the joint tight instead of threading into the top board.
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What a 1/4" lag screw is for
A 1/4" lag is a light-duty fastener: shelf and bracket hangers, fence and gate brackets, mounting a handrail bracket or a heavy shelf into a stud, and light timber hardware. It threads into framing lumber well and drives with a 7/16" wrench or socket.
It is not a heavy structural fastener. For a deck ledger, a beam-to-post joint, or anything holding real weight, step up to a 3/8" or 1/2" lag and follow the connector manufacturer's or your local code's specs.
Two holes, not one
A lag needs two holes: the pilot above, sized to the wood, plus a clearance hole the size of the bolt’s nominal diameter — for a 1/4" lag, a 1/4" clearance hole through the top piece. Drive with a wrench, socket or impact and finish by hand, so you don’t over-torque and snap the head; a washer under the head spreads the load.
Common questions
What size pilot hole for a 1/4 inch lag screw?
For a 1/4" lag screw, drill a 3/32" (2.4 mm) pilot in softwood, 5/32" (4 mm) in medium woods like Douglas fir, or 3/16" (4.8 mm) in hardwood. Also drill a 1/4" clearance hole for the smooth shank through the piece being fastened.
Why is the pilot hole bigger in hardwood than softwood for a 1/4" lag?
Because the woods behave differently. Hardwood is dense and splits easily, so a 1/4" lag needs a larger pilot (3/16", 4.8 mm) — sized close to the screw's root diameter — to relieve the pressure; too small and it cracks the board or shears the lag. Softwood fibres compress and grip around the threads, so a smaller pilot (3/32", 2.4 mm) still holds firmly without splitting. Same lag, different hole — matched to how much the wood can take.
Do lag screws need a pilot hole in softwood?
Yes — even in softwood a 1/4" lag needs a pilot hole (3/32", 2.4 mm). Softwood takes a smaller pilot than hardwood, but a lag this size can still split the board or shear off if you drive it dry.
What drill bit do I use for a 1/4" lag bolt?
It depends on the wood: a 3/32" bit for softwood, 5/32" for medium woods, and 3/16" for hardwood — matched to the lag's threaded portion. Then a 1/4" bit for the shank clearance hole.
Full lag screw pilot-hole 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 against Portland Bolt, Mudge Fasteners and Triangle Fastener’s spec sheet. On a split-prone board, stepping up one bit size is a reasonable call; structural work follows the 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.