Reference

Drill Size Chart

Decimal inch and millimeter equivalents for every number, letter, fractional, and common metric drill, plus which tap or clearance hole each size is used for.

Drill bits in the US come in four size systems: number drills (#80, the smallest at 0.0135 in, up to #1 at 0.228 in), letter drills (A at 0.234 in up to Z at 0.413 in), fractional drills in 1/64 in steps, and metric drills in millimeters. Number and letter drills fill the gaps between fractional sizes, which is why most tap drills and clearance drills on inch drawings are called out by number or letter.

Note the direction: number drills get smaller as the number goes up, while letter drills get larger from A to Z. The largest number drill (#1, 0.228 in) is just under the smallest letter drill (A, 0.234 in).

The number and letter tables also list the tap or clearance hole each drill is standard for. For the drill to use with a specific thread, see the tap drill size chart.

Number drill sizes (#1 to #80)

Wire-gauge number drills, from #1 (largest) to #80 (smallest). These are standard in ASME B94.11M.

Drill	Decimal (in)	Decimal (mm)	Standard use
#1	0.2280	5.79	#12 free clearance
#2	0.2210	5.61	#12 close clearance
#3	0.2130	5.41	1/4-28 tap
#4	0.2090	5.31	-
#5	0.2055	5.22	-
#6	0.2040	5.18	-
#7	0.2010	5.11	1/4-20 tap; #10 free clearance
#8	0.1990	5.05	-
#9	0.1960	4.98	#10 close clearance
#10	0.1935	4.91	-
#11	0.1910	4.85	-
#12	0.1890	4.80	-
#13	0.1850	4.70	-
#14	0.1820	4.62	#12-28 tap
#15	0.1800	4.57	-
#16	0.1770	4.50	#12-24 tap; #8 free clearance
#17	0.1730	4.39	-
#18	0.1695	4.31	#8 close clearance
#19	0.1660	4.22	-
#20	0.1610	4.09	-
#21	0.1590	4.04	#10-32 tap
#22	0.1570	3.99	-
#23	0.1540	3.91	-
#24	0.1520	3.86	-
#25	0.1495	3.80	#10-24 tap; #6 free clearance
#26	0.1470	3.73	-
#27	0.1440	3.66	#6 close clearance
#28	0.1405	3.57	-
#29	0.1360	3.45	#8-32 tap; #8-36 tap
#30	0.1285	3.26	#4 free clearance
#31	0.1200	3.05	-
#32	0.1160	2.95	#4 close clearance
#33	0.1130	2.87	#6-40 tap
#34	0.1110	2.82	-
#35	0.1100	2.79	-
#36	0.1065	2.71	#6-32 tap
#37	0.1040	2.64	#5-44 tap
#38	0.1015	2.58	#5-40 tap
#39	0.0995	2.53	-
#40	0.0980	2.49	-
#41	0.0960	2.44	#2 free clearance
#42	0.0935	2.37	#4-48 tap
#43	0.0890	2.26	#4-40 tap; #2 close clearance
#44	0.0860	2.18	-
#45	0.0820	2.08	#3-56 tap
#46	0.0810	2.06	-
#47	0.0785	1.99	#3-48 tap
#48	0.0760	1.93	-
#49	0.0730	1.85	-
#50	0.0700	1.78	#2-56 tap; #2-64 tap; #0 free clearance
#51	0.0670	1.70	-
#52	0.0635	1.61	#0 close clearance
#53	0.0595	1.51	#1-64 tap; #1-72 tap
#54	0.0550	1.40	-
#55	0.0520	1.32	-
#56	0.0465	1.18	-
#57	0.0430	1.09	-
#58	0.0420	1.07	-
#59	0.0410	1.04	-
#60	0.0400	1.02	-
#61	0.0390	0.99	-
#62	0.0380	0.97	-
#63	0.0370	0.94	-
#64	0.0360	0.91	-
#65	0.0350	0.89	-
#66	0.0330	0.84	-
#67	0.0320	0.81	-
#68	0.0310	0.79	-
#69	0.0292	0.74	-
#70	0.0280	0.71	-
#71	0.0260	0.66	-
#72	0.0250	0.64	-
#73	0.0240	0.61	-
#74	0.0225	0.57	-
#75	0.0210	0.53	-
#76	0.0200	0.51	-
#77	0.0180	0.46	-
#78	0.0160	0.41	-
#79	0.0145	0.37	-
#80	0.0135	0.34	-

Tap uses are for about 75% thread with a cutting tap. Clearance uses follow common close and free fit values for inch screws.

Letter drill sizes (A to Z)

Letter drills continue above the number series, from A (0.234 in) to Z (0.413 in).

Drill	Decimal (in)	Decimal (mm)	Standard use
A	0.2340	5.94	-
B	0.2380	6.05	-
C	0.2420	6.15	-
D	0.2460	6.25	-
E	0.2500	6.35	-
F	0.2570	6.53	5/16-18 tap; 1/4 close clearance
G	0.2610	6.63	-
H	0.2660	6.76	1/4 free clearance
I	0.2720	6.91	5/16-24 tap
J	0.2770	7.04	-
K	0.2810	7.14	-
L	0.2900	7.37	-
M	0.2950	7.49	-
N	0.3020	7.67	-
O	0.3160	8.03	-
P	0.3230	8.20	5/16 close clearance
Q	0.3320	8.43	3/8-24 tap; 5/16 free clearance
R	0.3390	8.61	-
S	0.3480	8.84	-
T	0.3580	9.09	-
U	0.3680	9.35	7/16-14 tap
V	0.3770	9.58	-
W	0.3860	9.80	3/8 close clearance
X	0.3970	10.08	3/8 free clearance
Y	0.4040	10.26	-
Z	0.4130	10.49	-

Fractional drill sizes (1/64 in to 1 in)

Fractional drills step in 1/64 in increments. Decimals are exact.

Fraction	Decimal (in)	Decimal (mm)
1/64"	0.015625	0.40
1/32"	0.03125	0.79
3/64"	0.046875	1.19
1/16"	0.0625	1.59
5/64"	0.078125	1.98
3/32"	0.09375	2.38
7/64"	0.109375	2.78
1/8"	0.125	3.18
9/64"	0.140625	3.57
5/32"	0.15625	3.97
11/64"	0.171875	4.37
3/16"	0.1875	4.76
13/64"	0.203125	5.16
7/32"	0.21875	5.56
15/64"	0.234375	5.95
1/4"	0.25	6.35
17/64"	0.265625	6.75
9/32"	0.28125	7.14
19/64"	0.296875	7.54
5/16"	0.3125	7.94
21/64"	0.328125	8.33
11/32"	0.34375	8.73
23/64"	0.359375	9.13
3/8"	0.375	9.53
25/64"	0.390625	9.92
13/32"	0.40625	10.32
27/64"	0.421875	10.72
7/16"	0.4375	11.11
29/64"	0.453125	11.51
15/32"	0.46875	11.91
31/64"	0.484375	12.30
1/2"	0.5	12.70
33/64"	0.515625	13.10
17/32"	0.53125	13.49
35/64"	0.546875	13.89
9/16"	0.5625	14.29
37/64"	0.578125	14.68
19/32"	0.59375	15.08
39/64"	0.609375	15.48
5/8"	0.625	15.88
41/64"	0.640625	16.27
21/32"	0.65625	16.67
43/64"	0.671875	17.07
11/16"	0.6875	17.46
45/64"	0.703125	17.86
23/32"	0.71875	18.26
47/64"	0.734375	18.65
3/4"	0.75	19.05
49/64"	0.765625	19.45
25/32"	0.78125	19.84
51/64"	0.796875	20.24
13/16"	0.8125	20.64
53/64"	0.828125	21.03
27/32"	0.84375	21.43
55/64"	0.859375	21.83
7/8"	0.875	22.23
57/64"	0.890625	22.62
29/32"	0.90625	23.02
59/64"	0.921875	23.42
15/16"	0.9375	23.81
61/64"	0.953125	24.21
31/32"	0.96875	24.61
63/64"	0.984375	25.00
1"	1	25.40

Metric drill sizes to inches

Common metric drills converted to inches, with the nearest fractional drill and the nearest number or letter drill. Metric tap drill sizes (1.6, 2.05, 3.3, 4.2, 6.8, 8.5, and 10.2 mm) are included.

Metric drill	Decimal (in)	Nearest fractional	Nearest number / letter
1.0 mm	0.0394	3/64" (0.0469)	#61 (0.0390)
1.25 mm	0.0492	3/64" (0.0469)	#56 (0.0465)
1.5 mm	0.0591	1/16" (0.0625)	#53 (0.0595)
1.6 mm	0.0630	1/16" (0.0625)	#52 (0.0635)
2.0 mm	0.0787	5/64" (0.0781)	#47 (0.0785)
2.05 mm	0.0807	5/64" (0.0781)	#46 (0.0810)
2.5 mm	0.0984	3/32" (0.0938)	#40 (0.0980)
2.9 mm	0.1142	7/64" (0.1094)	#33 (0.1130)
3.0 mm	0.1181	1/8" (0.1250)	#31 (0.1200)
3.3 mm	0.1299	1/8" (0.1250)	#30 (0.1285)
3.5 mm	0.1378	9/64" (0.1406)	#29 (0.1360)
4.0 mm	0.1575	5/32" (0.1563)	#22 (0.1570)
4.2 mm	0.1654	11/64" (0.1719)	#19 (0.1660)
4.5 mm	0.1772	11/64" (0.1719)	#16 (0.1770)
5.0 mm	0.1969	13/64" (0.2031)	#9 (0.1960)
5.5 mm	0.2165	7/32" (0.2188)	#3 (0.2130)
6.0 mm	0.2362	15/64" (0.2344)	B (0.2380)
6.5 mm	0.2559	1/4" (0.2500)	F (0.2570)
6.8 mm	0.2677	17/64" (0.2656)	H (0.2660)
7.0 mm	0.2756	9/32" (0.2813)	J (0.2770)
7.5 mm	0.2953	19/64" (0.2969)	M (0.2950)
8.0 mm	0.3150	5/16" (0.3125)	O (0.3160)
8.5 mm	0.3346	21/64" (0.3281)	Q (0.3320)
9.0 mm	0.3543	23/64" (0.3594)	T (0.3580)
9.5 mm	0.3740	3/8" (0.3750)	V (0.3770)
10.0 mm	0.3937	25/64" (0.3906)	X (0.3970)
10.2 mm	0.4016	13/32" (0.4063)	Y (0.4040)
10.5 mm	0.4134	13/32" (0.4063)	Z (0.4130)
11.0 mm	0.4331	7/16" (0.4375)	-
11.5 mm	0.4528	29/64" (0.4531)	-
12.0 mm	0.4724	15/32" (0.4688)	-
12.5 mm	0.4921	31/64" (0.4844)	-
13.0 mm	0.5118	33/64" (0.5156)	-
14.0 mm	0.5512	35/64" (0.5469)	-
15.0 mm	0.5906	19/32" (0.5938)	-
16.0 mm	0.6299	5/8" (0.6250)	-
17.0 mm	0.6693	43/64" (0.6719)	-
18.0 mm	0.7087	45/64" (0.7031)	-
19.0 mm	0.7480	3/4" (0.7500)	-
20.0 mm	0.7874	25/32" (0.7813)	-
21.0 mm	0.8268	53/64" (0.8281)	-
22.0 mm	0.8661	55/64" (0.8594)	-
23.0 mm	0.9055	29/32" (0.9063)	-
24.0 mm	0.9449	15/16" (0.9375)	-
25.0 mm	0.9843	63/64" (0.9844)	-

Nearest sizes are by diameter. A dash means the size is larger than the letter-drill range (Z, 0.413 in).

Number and letter drill decimals follow ASME B94.11M. Millimeter values are exact conversions (1 in = 25.4 mm) rounded to 0.01 mm.

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Frequently Asked Questions

What size is a #7 drill?

A #7 drill is 0.201 in (5.11 mm). It is the standard tap drill for a 1/4-20 UNC thread.

What size is a #29 drill?

A #29 drill is 0.136 in (3.45 mm). It is the tap drill for 8-32 UNC and 8-36 UNF threads.

What size is a #40 drill bit?

A #40 drill is 0.098 in (2.49 mm). It is close to a 2.5 mm drill (0.0984 in), which is the tap drill for M3 x 0.5.

What size is a letter F drill?

A letter F drill is 0.257 in (6.53 mm). It is the tap drill for 5/16-18 UNC and the close-fit clearance drill for a 1/4 in screw.

What size is an S drill bit?

A letter S drill is 0.348 in (8.84 mm).

What is 7/32 as a decimal?

7/32 in is 0.21875 in (5.56 mm). The nearest number drill is #2 (0.221 in).

What is a 6 mm drill in inches?

A 6 mm drill is 0.2362 in. The nearest fractional drill is 15/64 in (0.2344 in) and the nearest letter drill is B (0.238 in).

Which is bigger, a #1 drill or an A drill?

An A drill (0.234 in) is slightly bigger than a #1 drill (0.228 in). Number drills get smaller as the number rises, and letter drills get bigger from A to Z, so the letter series picks up just above #1.

Why do number and letter drills exist?

They fill the gaps between 1/64 in fractional steps. Many tap drills and clearance holes need a diameter between two fractions, such as the 0.201 in #7 drill for 1/4-20, so inch drawings often call out number and letter drills.

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