1/16" NPT tap threads - 1/4" drill bit.
1/8" NPT tap thteads - 21/64" drill bit.
1/4" NPT tap threads - 7/16" drill bit.
3/8" NPT tap threads - 9/16" drill bit.
1/2" NPT tap threads - 23/32" drill bit.
3/4" NPT tap threads - 29/32" drill bit.
1" NPT tap threads - 1 5/32" drill bit.