Pilot hole size for a 3/4" lag screw (or lag bolt)
The 3/4" lag screw — or 3/4" lag bolt — is about the heaviest lag in common use: for the biggest timber and beam connections, where the load is real and a smaller lag won't do.
A lag this thick generates enormous splitting force, so a correctly sized pilot hole is essential — skip it and you'll crack the timber or shear the head. Here's the pilot for a 3/4" lag in every wood type.
Lag bolt diameter
Change the diameter to compare sizes — ¼" through ¾".
Pilot hole for a 3/4" 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 3/4" (19 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 3/4" lag screw is for
The 3/4" lag is for the heaviest work: major beam and girder connections, large timber-frame and post-and-beam joints, heavy machinery and equipment mounts, and big retaining-wall and bridge-timber hardware. It drives with a 1-1/8" wrench or socket, always with a washer under the head.
This is heavy structural hardware. For any load-bearing connection, follow the fastener manufacturer's or an engineer's specification — the sizes here are a split-safe starting point, not an engineering spec.
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 3/4" lag, a 3/4" 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 3/4 inch lag screw?
For a 3/4" lag screw, drill a 13/32" (10.3 mm) pilot in softwood, 1/2" (12.7 mm) in medium woods like Douglas fir, or 9/16" (14.3 mm) in hardwood. Also drill a 3/4" clearance hole for the smooth shank through the piece being fastened.
Why is the pilot hole bigger in hardwood than softwood for a 3/4" lag?
Because the woods behave differently. Hardwood is dense and splits easily, so a 3/4" lag needs a larger pilot (9/16", 14.3 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 (13/32", 10.3 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 3/4" lag needs a pilot hole (13/32", 10.3 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 3/4" lag bolt?
It depends on the wood: a 13/32" bit for softwood, 1/2" for medium woods, and 9/16" for hardwood — matched to the lag's threaded portion. Then a 3/4" bit for the shank clearance hole.
Where these numbers come from
These follow common split-safe lag guidance cross-referenced across fastener references (Monster Bolts and a second agreeing chart), leaning toward not splitting the wood rather than 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.