Pilot hole size for a 5/8" lag screw (or lag bolt)
The 5/8" lag screw — or 5/8" lag bolt — is a heavy structural fastener, a step above the common 3/8" and 1/2" lags for big timber and beam connections that carry serious load.
At this diameter a pilot hole isn't optional: drive a 5/8" lag dry into hardwood or thick timber and you'll split the piece or snap the head. Here's the pilot for a 5/8" lag in every wood type.
Lag bolt diameter
Change the diameter to compare sizes — ¼" through ¾".
Pilot hole for a 5/8" 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 5/8" (15.9 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 5/8" lag screw is for
Reach for a 5/8" lag on heavy structural connections: large beam-to-post and post-to-beam joints, heavy timber and mantel mounts, big retaining-wall and landscape-timber work, and heavy-duty brackets and hangers. It drives with a 15/16" wrench or socket, and a washer under the head is standard to spread the load.
This is genuinely structural hardware. For load-bearing connections, treat these sizes as a starting point and follow the manufacturer's or an engineer's specification for the joint.
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 5/8" lag, a 5/8" 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 5/8 inch lag screw?
For a 5/8" lag screw, drill a 5/16" (7.9 mm) pilot in softwood, 13/32" (10.3 mm) in medium woods like Douglas fir, or 29/64" (11.5 mm) in hardwood. Also drill a 5/8" clearance hole for the smooth shank through the piece being fastened.
Why is the pilot hole bigger in hardwood than softwood for a 5/8" lag?
Because the woods behave differently. Hardwood is dense and splits easily, so a 5/8" lag needs a larger pilot (29/64", 11.5 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 (5/16", 7.9 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 5/8" lag needs a pilot hole (5/16", 7.9 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 5/8" lag bolt?
It depends on the wood: a 5/16" bit for softwood, 13/32" for medium woods, and 29/64" for hardwood — matched to the lag's threaded portion. Then a 5/8" 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.