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Pilot Hole Size Chart
Quick answer: Match the pilot to the screw’s root diameter in hardwood, one drill size under in softwood. A #8 takes 3/32″ in hardwood, 5/64″ in softwood, with an 11/64″ clearance hole through the top piece.
Pilot and clearance sizes for every screw gauge and lag size, split by wood species — plus the rule for working one out when your screw is not on the chart.
Pilot hole sizes by screw gauge
Two holes make a proper screw joint. The pilot hole goes into the receiving piece and gives the threads something to bite. The clearance hole goes through the top piece and lets the shank pass freely, so the head can pull the two boards tight. Skip the clearance hole and the screw threads hold the boards apart instead of together.
| Screw gauge | Diameter (in) | Pilot — softwood | Pilot — hardwood | Clearance hole |
|---|---|---|---|---|
| #0 | 0.060 | 1/64 | 1/32 | 1/16 |
| #1 | 0.073 | 1/32 | 1/32 | 5/64 |
| #2 | 0.086 | 1/32 | 3/64 | 3/32 |
| #3 | 0.099 | 3/64 | 1/16 | 7/64 |
| #4 | 0.112 | 3/64 | 1/16 | 7/64 |
| #5 | 0.125 | 1/16 | 5/64 | 1/8 |
| #6 | 0.138 | 1/16 | 5/64 | 9/64 |
| #7 | 0.151 | 1/16 | 3/32 | 5/32 |
| #8 | 0.164 | 5/64 | 3/32 | 11/64 |
| #9 | 0.177 | 5/64 | 7/64 | 3/16 |
| #10 | 0.190 | 3/32 | 7/64 | 3/16 |
| #12 | 0.216 | 7/64 | 1/8 | 7/32 |
| #14 | 0.242 | 7/64 | 9/64 | 1/4 |
| #16 | 0.268 | 9/64 | 5/32 | 17/64 |
| #18 | 0.294 | 9/64 | 3/16 | 19/64 |
| #20 | 0.320 | 11/64 | 13/64 | 21/64 |
| #24 | 0.372 | 3/16 | 7/32 | 3/8 |
Lag screw pilot holes
Lag screws need a stepped hole: full shank clearance for the unthreaded portion, then a smaller pilot for the threads. Lags are large enough that a wrong pilot either splits the timber or strips out.
| Lag size | Pilot — softwood | Pilot — hardwood | Shank clearance |
|---|---|---|---|
| 1/4 | 5/32 | 3/16 | 1/4 |
| 5/16 | 3/16 | 7/32 | 5/16 |
| 3/8 | 1/4 | 9/32 | 3/8 |
| 1/2 | 5/16 | 3/8 | 1/2 |
| 5/8 | 7/16 | 1/2 | 5/8 |
| 3/4 | 1/2 | 5/8 | 3/4 |
How to pick a pilot when the chart runs out
To find the root diameter on a screw that isn’t listed, hold a drill bit against it. The correct bit covers the solid core and leaves the thread crests visible on both sides. If the bit hides the threads entirely, it is too big and the screw will spin without pulling.
Species changes everything
| Species | Density | Pilot approach |
|---|---|---|
| Pine, fir, spruce, cedar | Soft | One size under root diameter; often no pilot away from edges |
| Poplar, alder | Medium | Root diameter |
| Oak, maple, ash, birch | Hard | Root diameter, always — and countersink flat heads |
| Ipe, cumaru, teak | Very hard | Root diameter plus a whisker; pre-drill every hole including deck faces |
| MDF, particleboard | Engineered | Root diameter; edges split readily, stay 1″ from the end |
Near an end grain cut, drill a pilot regardless of species. Wood splits along the grain, and the last two inches of a board have nothing holding the fibres together laterally.
Frequently asked questions
What size pilot hole for a #8 screw?
5/64 in (0.078 in) in softwood, 3/32 in (0.094 in) in hardwood. The clearance hole through the top piece is 11/64 in.
What size pilot hole for a 1/4 inch lag screw?
5/32 in in softwood, 3/16 in in hardwood, with a 1/4 in clearance hole through the unthreaded shank portion.
Do I need a pilot hole in pine?
Usually not in the middle of a board for screws #8 and under, but yes within about two inches of an end, and yes for anything #10 or larger. Pine splits along the grain readily at the ends.
What happens if the pilot hole is too big?
The threads have nothing to grip and the screw spins freely without drawing the joint tight. In structural connections this is a failure — back the screw out, glue a dowel or toothpick into the hole, and re-drill.