Pilot Hole & Drill Bit Chart
Look up the right pilot, clearance and countersink drill bit for any wood screw or tapped thread, in inches and millimetres.
- Pilot hole
- 3/32" (0.094" / 2.38 mm)
- Clearance hole
- 11/64" (0.172" / 4.37 mm)
- Countersink / head Ø
- 0.331" (8.41 mm)
- Fastener diameter
- 0.164" (4.17 mm)
Softwood pilot 3/32", hardwood pilot 7/64". Drill the clearance hole through the top piece only, then the pilot at least as deep as the threads that enter the bottom piece.
| Screw | Screw Ø | Softwood pilot | Hardwood pilot | Clearance (shank) | Head Ø (countersink) |
|---|
| Screw Ø | Softwood pilot | Hardwood pilot | Clearance (shank) | Head Ø (countersink) |
|---|
| Thread | Pitch | Tap drill | Clearance close | Clearance medium |
|---|
| Thread | Major Ø | Tap drill | Tap drill Ø | Clearance |
|---|
Sizes are the common workshop values: in hardwood the pilot hole matches the screw's root (core) diameter, in softwood it drops about one bit size. Test on an offcut of the same stock before you drill the real thing — species, moisture and how close you are to the end grain all change how much a board wants to split.
How it works
A pilot hole makes room for the screw's solid core so only the threads have to cut wood. Get it right and the screw drives easily and holds hard; get it wrong and you either split the board (hole too small) or strip the threads (hole too big). The rule this chart follows is the standard one: in hardwood drill the pilot at the screw's root diameter, and in softwood drop one bit size below that so the threads bite a little deeper. The clearance (shank) hole is drilled through the top piece only and is slightly larger than the screw's outer diameter, so the screw pulls the two boards together instead of jacking them apart. Countersink to the flat-head diameter shown in the last column.
Screw gauge converts to inches with Ø = 0.060 + 0.013 × gauge, so a #8 screw is
0.164″ across the threads, and the flat head is about 0.119 + 0.0265 × gauge, or
0.332″ for a #8. For machine threads the tap drill is the classic
tap drill = major diameter − pitch (ISO metric), which lands close to 75% thread
engagement — the sweet spot where you keep almost all the thread strength but roughly halve the
tapping torque. M6 × 1.0 therefore taps with a 5.0 mm bit, and ¼-20 UNC taps with a #7 drill
(0.201″). Every value on the page is computed and looked up in your browser — nothing is
uploaded, there is no sign-up and there are no ads.
Frequently asked questions
What size pilot hole do I need for a #8 wood screw?
Drill 3/32" (2.38 mm) in softwood and 7/64" (2.78 mm) in hardwood. The clearance (shank) hole through the top piece is 11/64" (4.37 mm), and a flat head countersinks to about 0.331" (8.4 mm). The rule behind those numbers is simple: in hardwood the pilot matches the screw's root diameter so only the threads cut wood, and in softwood you drop one bit size so the threads bite deeper. A #8 screw measures 0.060 + 0.013 × 8 = 0.164" over the threads.
What is the difference between a pilot hole and a clearance hole?
The pilot hole goes into the bottom piece and is sized to the screw's core so the threads still grip. The clearance hole goes all the way through the top piece and is slightly bigger than the screw's outside diameter, so the threads spin freely in it. Without a clearance hole the top board hangs on the threads and the two pieces are jacked apart instead of pulled together — the classic cause of a stubborn gap in a joint. Drill the clearance hole first, then the pilot through it.
How do I work out a tap drill size?
For ISO metric threads the standard is tap drill = major diameter − pitch, which gives roughly 75% thread engagement: M6 × 1.0 taps with a 5.0 mm bit, M8 × 1.25 with 6.8 mm, M10 × 1.5 with 8.5 mm. Unified threads use the published 75% table instead — 1/4-20 UNC takes a #7 drill (0.201") and #6-32 UNC takes a #36 (0.1065"). Seventy-five percent thread keeps nearly all the strength of a full thread while roughly halving the tapping torque, so taps are far less likely to snap.