Pilot hole size for a 3/8" lag screw (or lag bolt)
The 3/8" lag screw (or 3/8" lag bolt) is the workhorse — the most common lag for DIY structural and semi-structural work, big enough to carry a real load without needing a 1/2" monster.
It generates serious splitting force, so a correctly sized pilot is essential, especially in hardwood or near a board's end. Here's the pilot for a 3/8" lag in every wood type.
Lag bolt diameter
Change the diameter to compare sizes — ¼" through ¾".
Pilot hole for a 3/8" 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 3/8" (9.5 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 3/8" lag screw is for
The 3/8" lag shows up almost everywhere load matters: deck ledger boards (installed with the flashing and spacing your code requires), rim-to-joist connections, pergolas, playsets and swing sets, gate hinges, timber-frame joints, and heavy wall mounts — a big TV bracket or a heavy shelf standard driven into studs. It drives with a 9/16" wrench or socket.
For most home structural projects the 3/8" is the default. For truly heavy timber or beam connections, move up to 1/2"; for light brackets, a 1/4" is plenty.
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 3/8" lag, a 3/8" 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 3/8 inch lag screw?
For a 3/8" lag screw, drill a 11/64" (4.4 mm) pilot in softwood, 15/64" (6 mm) in medium woods like Douglas fir, or 1/4" (6.4 mm) in hardwood. Also drill a 3/8" clearance hole for the smooth shank through the piece being fastened.
Do lag screws need a pilot hole in softwood?
Yes — even in softwood a 3/8" lag needs a pilot hole (11/64", 4.4 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 3/8" lag bolt?
It depends on the wood: a 11/64" bit for softwood, 15/64" for medium woods, and 1/4" for hardwood — matched to the lag's threaded portion. Then a 3/8" 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.