Pilot hole size for a 5/16" lag screw (or lag bolt)
The 5/16" lag screw — or 5/16" lag bolt — sits between the light 1/4" lag and the workhorse 3/8": a bit more holding power for medium-duty brackets and hardware without stepping up to a full structural lag.
Even at this size a lag will split the wood or shear off if you drive it dry, so it needs a pilot hole. Here's the right pilot for a 5/16" lag in every wood type.
Lag bolt diameter
Change the diameter to compare sizes — ¼" through ¾".
Pilot hole for a 5/16" 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 5/16" (7.9 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 5/16" lag screw is for
A 5/16" lag suits medium-light hardware: heavier shelf and bracket mounts, fence and gate hardware, a sturdy handrail bracket, playset and light timber connections, and mounting into studs where a 1/4" feels marginal. It drives with a 1/2" wrench or socket.
It's still a light-to-medium fastener, not a heavy structural one. For deck ledgers, beam-to-post joints, or anything carrying real load, 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 5/16" lag, a 5/16" 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 5/16 inch lag screw?
For a 5/16" lag screw, drill a 9/64" (3.6 mm) pilot in softwood, 3/16" (4.8 mm) in medium woods like Douglas fir, or 7/32" (5.6 mm) in hardwood. Also drill a 5/16" clearance hole for the smooth shank through the piece being fastened.
Why is the pilot hole bigger in hardwood than softwood for a 5/16" lag?
Because the woods behave differently. Hardwood is dense and splits easily, so a 5/16" lag needs a larger pilot (7/32", 5.6 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 (9/64", 3.6 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 5/16" lag needs a pilot hole (9/64", 3.6 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 5/16" lag bolt?
It depends on the wood: a 9/64" bit for softwood, 3/16" for medium woods, and 7/32" for hardwood — matched to the lag's threaded portion. Then a 5/16" 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.