Quick answer
For wood screws, choose the pilot bit by screw size and wood hardness. Hardwood usually needs a slightly larger pilot hole than softwood, while tapered bits follow the screw shank more closely.
#6
7/64 in straight bit or 9/64 in tapered bit in hardwood; 3/32 in straight bit or 1/8 in tapered bit in softwood
#8
1/8 in straight bit or 11/64 in tapered bit in hardwood; 7/64 in straight bit or 5/32 in tapered bit in softwood
#10
9/64 in straight bit or 13/64 in tapered bit in hardwood; 1/8 in straight bit or 3/16 in tapered bit in softwood
#12
5/32 in straight bit or 7/32 in tapered bit in hardwood; 9/64 in straight bit or 13/64 in tapered bit in softwood
Common pilot hole sizes
#6: 7/64 in straight bit or 9/64 in tapered bit for hardwood, 3/32 in straight bit or 1/8 in tapered bit for softwood, with a 5/16 in countersink. #8: 1/8 in straight bit or 11/64 in tapered bit for hardwood, 7/64 in straight bit or 5/32 in tapered bit for softwood, with a 3/8 in countersink. #10: 9/64 in straight bit or 13/64 in tapered bit for hardwood, 1/8 in straight bit or 3/16 in tapered bit for softwood, with a 7/16 in countersink. #12: 5/32 in straight bit or 7/32 in tapered bit for hardwood, 9/64 in straight bit or 13/64 in tapered bit for softwood, with a 7/16 in countersink.
How to choose safely
If the screw is hard to drive or the wood starts to split, stop and step up slightly. If the screw spins without grabbing, the pilot hole is too large for that material.
Useful tools for this job
A combined drill-and-countersink bit set helps drill the pilot hole and recess the screw head in one step.
Frequently asked questions
Do I need a pilot hole for every wood screw?
Softwood can sometimes take small screws without a pilot hole, but pilot holes reduce splitting and make alignment easier, especially near board ends.
Should the drill bit match the screw threads?
For wood screws, the pilot hole usually matches the screw core more than the outside thread diameter so the threads can still bite.