Quick answer
For a #12 wood screw, use a 5/32 inch straight bit in hardwood or a 9/64 inch straight bit in softwood. With tapered bits, use 7/32 inch for hardwood or 13/64 inch for softwood.
Hardwood straight bit
5/32 in
Softwood straight bit
9/64 in
Hardwood tapered bit
7/32 in
Softwood tapered bit
13/64 in
Why the pilot hole matters
#12 screws are large enough to split boards or overload a driver if the pilot hole is too small.
Countersink size
A 7/16 inch countersink is a common match for a flat-head #12 wood screw.
Useful tools for this job
For #12 screws, a sturdy countersink bit and driver bits are good companion purchases because the installation force is higher.
Frequently asked questions
Should I drill deeper than the screw length?
No. Drill the pilot hole to the screw length or slightly shorter, unless the project instructions say otherwise.