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Tap drill chart (printable)

Read the full tap drill guide

UNC (coarse)

Tap	Drill	in
#4-40	#43	0.0890
#6-32	#36	0.1065
#8-32	#29	0.1360
#10-24	#25	0.1495
#12-24	#16	0.1770
1/4-20	#7	0.2010
5/16-18	F	0.2570
3/8-16	5/16	0.3125
7/16-14	U	0.3680
1/2-13	27/64	0.4219
9/16-12	31/64	0.4844
5/8-11	17/32	0.5313
3/4-10	21/32	0.6563
7/8-9	49/64	0.7656
1-8	7/8	0.8750

UNF (fine)

Tap	Drill	in
#4-48	#42	0.0935
#6-40	#33	0.1130
#8-36	#29	0.1360
#10-32	#21	0.1590
#12-28	#14	0.1820
1/4-28	#3	0.2130
5/16-24	I	0.2720
3/8-24	Q	0.3320
7/16-20	25/64	0.3906
1/2-20	29/64	0.4531
9/16-18	33/64	0.5156
5/8-18	37/64	0.5781
3/4-16	11/16	0.6875
7/8-14	13/16	0.8125
1-12	59/64	0.9219

Metric coarse

Tap	Drill	mm
M3 x 0.5	2.5 mm	2.5
M4 x 0.7	3.3 mm	3.3
M5 x 0.8	4.2 mm	4.2
M6 x 1.0	5.0 mm	5.0
M8 x 1.25	6.8 mm	6.8
M10 x 1.5	8.5 mm	8.5
M12 x 1.75	10.2 mm	10.2
M14 x 2.0	12.0 mm	12.0
M16 x 2.0	14.0 mm	14.0
M20 x 2.5	17.5 mm	17.5

About 75% thread. For hard metals a slightly larger drill (60 to 65% thread) is easier to tap. thehometoolspro.com/drill-bits/tap-sizes/