Drill size for pipe taps

                     ANSI Pipe Taps
               (for 75% depth of thread)
Pipe Tap Size       Drill Size NPT     Drill Size NPS
1/16"-27                  C                          1/4"
1/8"-27                    R                          S
1/4"-18                    7/16"                    29/64"
3/8"-18                    37/64"                  19/32"
1/2"-14                    23/32"                  47/64"
3/4"-14                    59/64"                  15/16"
1"-11-1/2                  1-5/32"                 1-3/16"
1-1/4"-11-1/2            1-1/2"                    1-33/64"
1-1/2"-11-1/2            1-47/64"                1-3/4"
2"-11-1/2                 2-7/32"                 2-7/32"
2-1/2"-8                   2-5/8"                   2-21/32"
3"-8                         3-1/4"                   3-9/32"
3-1/2"-8                   3-3/4"                    3-25/32"
4"-8                         4-1/4"                    4-9/32"

5''-8                        5-9/32''

6''-8                        6-11/32''