Based on approximately 75% of full thread
TAP   THREAD FORM TAP DRILL SIZE
#0 - 80 NF 3/64
#1 - 64 NC no. 53
#1 - 72 NF no. 53
#2 - 56 NC no. 50
#2 - 64 NF no. 50
#3 - 48 NC no. 47
#3 - 56 NF no. 45
#4 - 36 NS no. 42
#4 - 40 NC no. 43
#4 - 48 NF no. 44
#5 - 40 NC no. 38
#5 - 44 NF no. 37
#6 - 32 NC no. 36
#6 - 40 NF no. 33
#8 - 32 NC no. 29
#8 - 36 NF no. 29
#10 - 24 NC no. 25
#10 - 32 NF no. 21
#12 - 24 NC no. 16
#12 - 28 NF no. 14
1/4 - 20 NC no. 7
1/4 - 28 NF no. 3
5/16 - 18 NC F
5/16 - 24 NF I
3/8 - 16 NC 5/16
3/8 - 24 NF Q
7/16 - 14 NC U
7/16 - 20 NF 25/64
1/2 - 13 NC 27/64
1/2 - 20 NF 29/64
9/16 - 12 NC 31/64
9/16 - 18 NF 33/64
5/8 - 11 NC 17/32
5/8 - 18 NF 37/64
11/16 - 11 NS 19/32
11/16 - 16 NS 5/8
3/4 - 10 NC 21/32
3/4 - 16 NF 11/16
7/8 - 9 NC 49/64
7/8 - 14 NF 13/64
1 - 8 NC 7/8
1 - 12 NF 15/16
1 - 14 NS 59/64
1-1/8 - 7 NC 63/64
1-1/8 - 12 NF 1-3/64
1-1/4 - 7 NC 1-7/64
1-1/4 - 12 NF 1-11/64
1-1/2 - 6 NC 1-11/32
1-1/2 - 12 NF 1-27/64
1-3/4 - 5 NC 1-9/16
2 - 4-1/2 NC 1-25/32
2-1/4 - 4-1/2 NC 2-1/32
2-1/2 - 4 NC 2-1/4
3 - 4 NC 2-3/4
3-1/2 - 4 NC 3-1/4

THREAD FORM SYMBOLS
NC - American National Coarse Thread Series
NF - American National Fine Thread Series
NS - American National Thread Special

A formula can be used to find the nearest standard tap drill size for any size tap.
Click here for a tap drill size calculator.