Number drills run from #80 (0.0135 in) up to #1 (0.228 in), letter drills from A (0.234 in) to Z (0.413 in), and fractional drills step by 1/64 in, so every size from #80 to 1 in has an exact decimal equivalent.
Drill bit sizes: fractional, number, letter and metric with decimal inch and mm
Fractional drills, 1/64 in to 1 in
Drill size
Decimal (in)
Metric (mm)
1/64
0.015625
0.397
1/32
0.03125
0.794
3/64
0.046875
1.191
1/16
0.0625
1.587
5/64
0.078125
1.984
3/32
0.09375
2.381
7/64
0.109375
2.778
1/8
0.125
3.175
9/64
0.140625
3.572
5/32
0.15625
3.969
11/64
0.171875
4.366
3/16
0.1875
4.762
13/64
0.203125
5.159
7/32
0.21875
5.556
15/64
0.234375
5.953
1/4
0.25
6.35
17/64
0.265625
6.747
9/32
0.28125
7.144
19/64
0.296875
7.541
5/16
0.3125
7.938
21/64
0.328125
8.334
11/32
0.34375
8.731
23/64
0.359375
9.128
3/8
0.375
9.525
25/64
0.390625
9.922
13/32
0.40625
10.319
27/64
0.421875
10.716
7/16
0.4375
11.112
29/64
0.453125
11.509
15/32
0.46875
11.906
31/64
0.484375
12.303
1/2
0.5
12.7
33/64
0.515625
13.097
17/32
0.53125
13.494
35/64
0.546875
13.891
9/16
0.5625
14.287
37/64
0.578125
14.684
19/32
0.59375
15.081
39/64
0.609375
15.478
5/8
0.625
15.875
41/64
0.640625
16.272
21/32
0.65625
16.669
43/64
0.671875
17.066
11/16
0.6875
17.462
45/64
0.703125
17.859
23/32
0.71875
18.256
47/64
0.734375
18.653
3/4
0.75
19.05
49/64
0.765625
19.447
25/32
0.78125
19.844
51/64
0.796875
20.241
13/16
0.8125
20.637
53/64
0.828125
21.034
27/32
0.84375
21.431
55/64
0.859375
21.828
7/8
0.875
22.225
57/64
0.890625
22.622
29/32
0.90625
23.019
59/64
0.921875
23.416
15/16
0.9375
23.812
61/64
0.953125
24.209
31/32
0.96875
24.606
63/64
0.984375
25.003
1
1
25.4
Number (wire gauge) drills, #1 to #80
Drill size
Decimal (in)
Metric (mm)
#1
0.228
5.791
#2
0.221
5.613
#3
0.213
5.41
#4
0.209
5.309
#5
0.2055
5.22
#6
0.204
5.182
#7
0.201
5.105
#8
0.199
5.055
#9
0.196
4.978
#10
0.1935
4.915
#11
0.191
4.851
#12
0.189
4.801
#13
0.185
4.699
#14
0.182
4.623
#15
0.18
4.572
#16
0.177
4.496
#17
0.173
4.394
#18
0.1695
4.305
#19
0.166
4.216
#20
0.161
4.089
#21
0.159
4.039
#22
0.157
3.988
#23
0.154
3.912
#24
0.152
3.861
#25
0.1495
3.797
#26
0.147
3.734
#27
0.144
3.658
#28
0.1405
3.569
#29
0.136
3.454
#30
0.1285
3.264
#31
0.12
3.048
#32
0.116
2.946
#33
0.113
2.87
#34
0.111
2.819
#35
0.11
2.794
#36
0.1065
2.705
#37
0.104
2.642
#38
0.1015
2.578
#39
0.0995
2.527
#40
0.098
2.489
#41
0.096
2.438
#42
0.0935
2.375
#43
0.089
2.261
#44
0.086
2.184
#45
0.082
2.083
#46
0.081
2.057
#47
0.0785
1.994
#48
0.076
1.93
#49
0.073
1.854
#50
0.07
1.778
#51
0.067
1.702
#52
0.0635
1.613
#53
0.0595
1.511
#54
0.055
1.397
#55
0.052
1.321
#56
0.0465
1.181
#57
0.043
1.092
#58
0.042
1.067
#59
0.041
1.041
#60
0.04
1.016
#61
0.039
0.991
#62
0.038
0.965
#63
0.037
0.94
#64
0.036
0.914
#65
0.035
0.889
#66
0.033
0.838
#67
0.032
0.813
#68
0.031
0.787
#69
0.0292
0.742
#70
0.028
0.711
#71
0.026
0.66
#72
0.025
0.635
#73
0.024
0.61
#74
0.0225
0.571
#75
0.021
0.533
#76
0.02
0.508
#77
0.018
0.457
#78
0.016
0.406
#79
0.0145
0.368
#80
0.0135
0.343
Letter drills, A to Z
Drill size
Decimal (in)
Metric (mm)
A
0.234
5.944
B
0.238
6.045
C
0.242
6.147
D
0.246
6.248
E
0.25
6.35
F
0.257
6.528
G
0.261
6.629
H
0.266
6.756
I
0.272
6.909
J
0.277
7.036
K
0.281
7.137
L
0.29
7.366
M
0.295
7.493
N
0.302
7.671
O
0.316
8.026
P
0.323
8.204
Q
0.332
8.433
R
0.339
8.611
S
0.348
8.839
T
0.358
9.093
U
0.368
9.347
V
0.377
9.576
W
0.386
9.804
X
0.397
10.084
Y
0.404
10.262
Z
0.413
10.49
Metric drills, 0.5 mm to 13 mm
Drill size
Decimal (in)
Metric (mm)
0.5 mm
0.0197
0.5
0.6 mm
0.0236
0.6
0.7 mm
0.0276
0.7
0.8 mm
0.0315
0.8
0.9 mm
0.0354
0.9
1.0 mm
0.0394
1
1.1 mm
0.0433
1.1
1.2 mm
0.0472
1.2
1.3 mm
0.0512
1.3
1.4 mm
0.0551
1.4
1.5 mm
0.0591
1.5
1.6 mm
0.063
1.6
1.7 mm
0.0669
1.7
1.8 mm
0.0709
1.8
1.9 mm
0.0748
1.9
2.0 mm
0.0787
2
2.1 mm
0.0827
2.1
2.2 mm
0.0866
2.2
2.3 mm
0.0906
2.3
2.4 mm
0.0945
2.4
2.5 mm
0.0984
2.5
2.6 mm
0.1024
2.6
2.7 mm
0.1063
2.7
2.8 mm
0.1102
2.8
2.9 mm
0.1142
2.9
3.0 mm
0.1181
3
3.1 mm
0.122
3.1
3.2 mm
0.126
3.2
3.3 mm
0.1299
3.3
3.4 mm
0.1339
3.4
3.5 mm
0.1378
3.5
3.6 mm
0.1417
3.6
3.7 mm
0.1457
3.7
3.8 mm
0.1496
3.8
3.9 mm
0.1535
3.9
4.0 mm
0.1575
4
4.1 mm
0.1614
4.1
4.2 mm
0.1654
4.2
4.3 mm
0.1693
4.3
4.4 mm
0.1732
4.4
4.5 mm
0.1772
4.5
4.6 mm
0.1811
4.6
4.7 mm
0.185
4.7
4.8 mm
0.189
4.8
4.9 mm
0.1929
4.9
5.0 mm
0.1969
5
5.1 mm
0.2008
5.1
5.2 mm
0.2047
5.2
5.3 mm
0.2087
5.3
5.4 mm
0.2126
5.4
5.5 mm
0.2165
5.5
5.6 mm
0.2205
5.6
5.7 mm
0.2244
5.7
5.8 mm
0.2283
5.8
5.9 mm
0.2323
5.9
6.0 mm
0.2362
6
6.1 mm
0.2402
6.1
6.2 mm
0.2441
6.2
6.3 mm
0.248
6.3
6.4 mm
0.252
6.4
6.5 mm
0.2559
6.5
6.6 mm
0.2598
6.6
6.7 mm
0.2638
6.7
6.8 mm
0.2677
6.8
6.9 mm
0.2717
6.9
7.0 mm
0.2756
7
7.1 mm
0.2795
7.1
7.2 mm
0.2835
7.2
7.3 mm
0.2874
7.3
7.4 mm
0.2913
7.4
7.5 mm
0.2953
7.5
7.6 mm
0.2992
7.6
7.7 mm
0.3031
7.7
7.8 mm
0.3071
7.8
7.9 mm
0.311
7.9
8.0 mm
0.315
8
8.1 mm
0.3189
8.1
8.2 mm
0.3228
8.2
8.3 mm
0.3268
8.3
8.4 mm
0.3307
8.4
8.5 mm
0.3346
8.5
8.6 mm
0.3386
8.6
8.7 mm
0.3425
8.7
8.8 mm
0.3465
8.8
8.9 mm
0.3504
8.9
9.0 mm
0.3543
9
9.1 mm
0.3583
9.1
9.2 mm
0.3622
9.2
9.3 mm
0.3661
9.3
9.4 mm
0.3701
9.4
9.5 mm
0.374
9.5
9.6 mm
0.378
9.6
9.7 mm
0.3819
9.7
9.8 mm
0.3858
9.8
9.9 mm
0.3898
9.9
10.0 mm
0.3937
10
10.2 mm
0.4016
10.2
10.5 mm
0.4134
10.5
11.0 mm
0.4331
11
11.5 mm
0.4528
11.5
12.0 mm
0.4724
12
12.5 mm
0.4921
12.5
13.0 mm
0.5118
13
ASME B94.11M is the US standard for twist drill sizes and tolerances. The number and letter decimals here are Kennametal's; every published chart checked uses the same values apart from the printing errors noted in the cross-checks.
Fractional values are exact (n/64 in). Metric drills are listed in 0.1 mm steps to 10 mm, then 10.2 mm (the M12 x 1.75 tap drill) and 0.5 mm steps to 13 mm.
mm values for inch drills and inch values for metric drills are computed from 25.4 mm per inch and rounded to 3 and 4 decimals.
Drill diameter is not hole size. A twist drill generally cuts slightly oversize (a Government of India machinist training text allows 0.05 mm); drill undersize and ream when the hole size matters.
Fractional: 1/64 in steps. 1/64 in is 0.015625 in; 1/2 in is 0.500 in.
Number (wire gauge): #80 is the smallest at 0.0135 in and #1 the largest at 0.228 in. The number goes down as the drill gets bigger. Kennametal's chart continues the series below #80, down to #97 at 0.0059 in.
Letter: A (0.234 in) to Z (0.413 in), picking up just above #1.
Metric: sized directly in millimeters. Fastenal's chart lists them in 0.05 mm steps from 1 to 2.5 mm, 0.1 mm steps from there to 10 mm, and wider steps above.
ASME B94.11M is the US standard for twist drills, covering sizes and tolerances in inch and metric. Every chart checked for this page uses the same decimals for number and letter drills, apart from a few printing errors noted with the table.
How the systems fit together
Number and letter drills fall between the 1/64 in fractional steps, and tap drill charts use many of them. Around 1/4 in the sizes interleave like this:
#1 = 0.228 in, A = 0.234 in, 15/64 = 0.2344 in
E = 0.250 in, the same as 1/4 in
F = 0.257 in, G = 0.261 in, 17/64 = 0.2656 in, H = 0.266 in
Some sizes nearly coincide. A 5.1 mm drill (0.2008 in) is within 0.0002 in of a #7 (0.201 in), the 1/4-20 tap drill. A 6.8 mm drill (0.2677 in), the M8 tap drill, sits between H (0.266 in) and I (0.272 in).
Converting between inch and metric
One inch is exactly 25.4 mm (NIST SP 811). To convert a drill size:
Inch to mm: multiply by 25.4. A #29 drill (0.136 in) is 3.454 mm.
mm to inch: divide by 25.4. A 10.2 mm drill is 0.4016 in.
The table gives mm to three decimals for inch drills and inches to four decimals for metric drills.
Common uses of number and letter drills
Many everyday tap drills are number or letter sizes. Little Machine Shop's tap drill chart lists, for example:
#43 for 4-40, #36 for 6-32, #29 for 8-32, #25 for 10-24 and #21 for 10-32
#7 for 1/4-20, F for 5/16-18 and U for 7/16-14
Clearance: F (0.257 in) for a close fit on a 1/4 in screw, H (0.266 in) for a free fit
The tap drill chart gives the drill, its decimal size and the thread percentage for every UNC, UNF and metric size it covers.
Drill size is not hole size
A twist drill generally cuts a hole a little larger than its own diameter. A Government of India machinist training text allows 0.05 mm of oversize for every drill diameter when working out the drill for a reamed hole.
When a hole has to be on size, drill undersize and finish with a reamer. The same text leaves 0.1 to 0.2 mm for the reamer on holes under 5 mm, 0.2 to 0.3 mm from 5 to 20 mm and 0.3 to 0.5 mm from 21 to 50 mm, so a 10 mm reamed hole in mild steel starts with a 9.75 mm drill. For tapped holes, the slight oversize is one reason actual thread engagement can run a few points below the chart percentage.