OneTaskBy Tim King · Updated
Pilot Hole Size Chart
Pilot Hole Size Chart, Tap Drill and Clearance Sizes
OneTask is free, with no sign-up, and built by one person.
If this tool saved you time, a coffee helps keep new ones coming.
How the Screw and Bolt Size Chart works
This pilot hole size chart gives the drill bit to use for a wood screw in softwood and hardwood, and for bolts the thread pitch, the tap drill for cutting a thread, the clearance hole for the bolt to pass through and the spanner size. Pick a gauge from #2 to #14, a metric bolt from M3 to M20, or a UNC bolt from 1/4 to 5/8 inch.
A pilot hole guides a wood screw and stops the timber splitting. It should be about the size of the screw's core (the solid shaft between the threads), a little smaller in softwood and a little larger in dense hardwood. A clearance hole in the top piece lets the screw pull the two pieces together instead of jacking them apart.
How to use the pilot hole size chart
The rule of thumb formula for a tap drill is: tap drill = nominal diameter − thread pitch. For example, an M8 bolt has a 1.25 mm pitch, so the tap drill is 8 − 1.25 = 6.75, rounded to a 6.8 mm bit; the medium-fit clearance hole is 9 mm and the spanner is 13 mm. A second example: a #8 wood screw has a 4.2 mm shank and takes a 2.4 mm pilot in softwood, a 3.2 mm pilot in hardwood and a 4.5 mm clearance hole.
Wood screw gauges are an old imperial numbering, still used on packets in the UK and US; many metric screws are sold by diameter instead, so a 4 mm screw sits between #8 and #10. For very small screws in soft timber you can often skip the pilot and use a bradawl. Always test on an offcut to check how many turns the screw needs and whether the wood splits.
Pilot, tap drill and clearance hole sizes for common screws and bolts
| Size | Type | Pilot or tap drill | Clearance hole |
|---|---|---|---|
| #2 | Wood screw | 1.2 mm softwood, 1.6 mm hardwood | 2.4 mm |
| #4 | Wood screw | 1.6 mm softwood, 2 mm hardwood | 3 mm |
| #6 | Wood screw | 2 mm softwood, 2.4 mm hardwood | 3.8 mm |
| #8 | Wood screw | 2.4 mm softwood, 3.2 mm hardwood | 4.5 mm |
| #10 | Wood screw | 2.8 mm softwood, 3.5 mm hardwood | 5 mm |
| #12 | Wood screw | 3.2 mm softwood, 4 mm hardwood | 6 mm |
| #14 | Wood screw | 3.5 mm softwood, 4.5 mm hardwood | 6.5 mm |
| M3 | Metric bolt, 0.5 mm pitch | 2.5 mm tap | 3.4 mm |
| M4 | Metric bolt, 0.7 mm pitch | 3.3 mm tap | 4.5 mm |
| M5 | Metric bolt, 0.8 mm pitch | 4.2 mm tap | 5.5 mm |
| M6 | Metric bolt, 1 mm pitch | 5 mm tap | 6.6 mm |
| M8 | Metric bolt, 1.25 mm pitch | 6.8 mm tap | 9 mm |
| M10 | Metric bolt, 1.5 mm pitch | 8.5 mm tap | 11 mm |
| M12 | Metric bolt, 1.75 mm pitch | 10.2 mm tap | 13.5 mm |
| M16 | Metric bolt, 2 mm pitch | 14 mm tap | 17.5 mm |
| M20 | Metric bolt, 2.5 mm pitch | 17.5 mm tap | 22 mm |
| 1/4 in | UNC bolt, 20 TPI | #7 (0.201 in) tap | 17/64 in |
| 5/16 in | UNC bolt, 18 TPI | F (0.257 in) tap | 21/64 in |
| 3/8 in | UNC bolt, 16 TPI | 5/16 in tap | 25/64 in |
| 1/2 in | UNC bolt, 13 TPI | 27/64 in tap | 17/32 in |
| 5/8 in | UNC bolt, 11 TPI | 17/32 in tap | 21/32 in |
FAQ
What size pilot hole for a #8 screw?
Use a 2.4 mm (3/32 in) bit in softwood and a 3.2 mm (1/8 in) bit in hardwood. If the top piece needs to clamp down tight, drill it with a 4.5 mm (11/64 in) clearance hole so the thread only grips in the bottom piece.
What size drill bit for an M6 tap?
Use a 5.0 mm drill for an M6 coarse thread, which has a 1.0 mm pitch. The general rule is the bolt diameter minus the pitch, which gives about 75% thread engagement and is strong enough for most uses without breaking taps.
What is a clearance hole?
A clearance hole is drilled slightly bigger than the bolt or screw so it slides through without engaging. It lets the fastener clamp parts together. Medium-fit clearance holes, like M8 in a 9 mm hole, give a little room to line parts up.
See also: all DIY & Building tools, Everyday Maths, Loans & Savings, Dates & Time, Health, Home & DIY, Unit Converters, Education, Science, Creative, Garden & Farm, Gaming & PC, Work & Business, Fun, Maps & GPS, Cost Estimators, Sport & Clubs.
Related tools
Board Foot Calculator
Board feet, cubic volume and cost of timber or lumber.
Beam Load Calculator
Bending stress and deflection of a beam under an even load.
Decking Calculator
Decking boards, lineal metres, joists and screws for a deck, with waste.
Fence Calculator
Posts, rails, pickets and concrete for a fence run, with waste.
Length Converter
Convert between metric and imperial lengths.