Quick answer
A wood screw pilot hole should usually match the screw core, not the outside thread. For common straight bits: #6 uses 7/64 inch in hardwood, #8 uses 1/8 inch, #10 uses 9/64 inch, and #12 uses 5/32 inch.
#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
Quick chart
#6: 7/64 in straight bit or 9/64 in tapered bit for hardwood and 3/32 in straight bit or 1/8 in tapered bit for softwood. #8: 1/8 in straight bit or 11/64 in tapered bit for hardwood and 7/64 in straight bit or 5/32 in tapered bit for softwood. #10: 9/64 in straight bit or 13/64 in tapered bit for hardwood and 1/8 in straight bit or 3/16 in tapered bit for softwood. #12: 5/32 in straight bit or 7/32 in tapered bit for hardwood and 9/64 in straight bit or 13/64 in tapered bit for softwood.
Hardwood versus softwood
Hardwood needs a slightly larger pilot hole because it compresses less and splits more easily.
Useful tools for this job
A pilot-hole and countersink set is the best next step because it solves both splitting and flush screw heads.
Frequently asked questions
Should a pilot hole be as wide as the screw?
No. For wood screws, the pilot hole should allow the screw core to pass while leaving the threads enough wood to bite.