What size pilot hole for a lag screw?
Pick your lag bolt diameter and wood type — get the pilot hole size, plus the shank clearance hole.
Lag bolt diameter
Nominal bolt diameter — ¼" through ¾".
Wood type
Drill this pilot
For a 3/8" lag in softwood
3/8" shank clearance hole (top piece)
Soft and forgiving — the smaller pilot still grips well.
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Two holes, not one
A lag needs two holes. The pilot hole above is for the threaded portion, drilled deep into the receiving piece. Then drill a clearance hole the size of the bolt’s nominal diameter (a ⅜" lag → a ⅜" hole) through the top piece being fastened, so the smooth shank slips through and the lag pulls the joint tight instead of threading into the top board.
Installing lag screws
- Center the lag in the middle third of the stud or timber — avoid end grain, which holds poorly.
- Drive with a wrench, socket, or impact, then finish by hand — over-torquing can strip the hole or snap the head.
- Add a washer under the head to spread the load.
- Outdoors, use hot-dipped galvanized or stainless lags so they don’t corrode.
Where these numbers come from
Pilot sizes for lag bolts vary by source and wood type — these follow common split-safe guidance cross-referenced across fastener references (Monster Bolts and a second agreeing chart). They lean toward not splitting the wood, which is the usual DIY concern, rather than squeezing out 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.