This guide is for cutting taps only!  Do not use these drill sizes for Thread Forming Taps!!!! 

Cutting Taps based on approx. 75% Thread.

Tap Size     Drill Size

0-80               3/64

1-64               #53

1-72               #53

2-56               #50

2-64               #50

3-48               #47

3-56               #46

4-40               #43

4-48               #42

5-40               #39

5-44               #38

6-32               #36

6-40               #33

8-32               #29

8-36               #29

10-24             #25

10-32             #21

12-24             #17

12-28             #15

1/4-20            #7

1/4-28            #3

5/16-18          F

5/16-24          I

3/8-16            5/16

3/8-24            Q

7/16-14          U

7/16-20          W

1/2-13            27/64

1/2-20            29/64

9/16-12          31/64

9/16-18          33/64

5/8-11            17/32

5/8-18            37/64

3/4-10            21/32

3/4-16            11/16

7/8-9              49/64

7/8-14            13/16

1-8                 7/8

1-12               59/64

1-14               15/16