Letter Sizes Series J100
General purpose drills for steels, cast steel, grey cast iron, malleable cast iron, nickel brass, aluminium alloy and graphite. Point angle: 118, helix angle: normal, web thickness: standard, relief angle: 8 - 20, tolerance of diameter: h8, tolerance of shank: f11. Manufactured to ANSI B328, BS 328 and DIN 338.


Number & Letter Gauge Drills
Gauge drills are still in common use in the U.S. In the past, they were popular elsewhere, but now have been largely discarded in favour of metric sizes. When using a gauge rather than a standard measurement of size the user has to refer to a table of gauge sizes. For example, to drill a hole which is 10 thou (thousandth of an inch = .0010) bigger than a 1/8 inch bolt, the user must first work out that 1/8 inch is
125 thou, so the hole size needed is 135 thou, and then must look up in a reference book which gauge drill bit is closest to this diameter - a number 29.
Number gauge sizes are similar or equivalent in some respects to, but different from, American wire gauge.
Number gauge is usually used from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). Number gauge is actually defined down to size 97, but these smaller sizes are rarely encountered. It happens that as the technology for making small drill bits and drilling small holes has become more available,
metric drills have become the norm.
The gauge-to-diameter conversion does not follow a set formula, but rather was defined as a useful and practical measure. The graph shows how gauge diameters change with gauge. Each step along the horizontal axis is one gauge size.
The step size between adjacent gauges is smaller for smaller gauges. This is appropriate, because the tolerance of the diameter of drilled holes is closer for smaller drill bits.

Gauge inch mm
80 0.014 0.343
79 0.015 0.368
78 0.016 0.406
77 0.018 0.457
76 0.020 0.508
75 0.021 0.533
74 0.023 0.572
73 0.024 0.610
72 0.025 0.635
71 0.026 0.660
70 0.028 0.711
69 0.029 0.742
68 0.031 0.787
67 0.032 0.813
66 0.033 0.838
65 0.035 0.889
64 0.036 0.914
63 0.037 0.940
62 0.038 0.965
61 0.039 0.991
60 0.040 1.016
59 0.041 1.041
58 0.042 1.067
57 0.043 1.092
56 0.046 1.181
55 0.052 1.321
54 0.055 1.397
53 0.059 1.511
52 0.064 1.613
51 0.067 1.702
50 0.070 1.778
49 0.073 1.854
48 0.076 1.930
47 0.079 1.994
46 0.081 2.057
45 0.082 2.083
44 0.086 2.184
43 0.089 2.261
42 0.094 2.375
41 0.096 2.438
40 0.098 2.489
39 0.099 2.527
38 0.101 2.578
37 0.104 2.642
36 0.106 2.705
35 0.110 2.794
34 0.111 2.819
33 0.113 2.870
32 0.116 2.946
31 0.120 3.048
30 0.129 3.264
29 0.136 3.454
28 0.141 3.569
27 0.144 3.658
26 0.147 3.734
25 0.149 3.797
24 0.152 3.861
23 0.154 3.912
22 0.157 3.988
21 0.159 4.039
20 0.161 4.089
19 0.166 4.216
18 0.169 4.305
17 0.173 4.394
16 0.177 4.496
15 0.180 4.572
14 0.182 4.623
13 0.185 4.699
12 0.189 4.801
11 0.191 4.851
10 0.194 4.915
9 0.196 4.978
8 0.199 5.055
7 0.201 5.105
6 0.204 5.182
5 0.206 5.220
4 0.209 5.309
3 0.213 5.410
2 0.221 5.613
1 0.228 5.791

A 0.234 5.944
B 0.238 6.045
C 0.242 6.147
D 0.246 6.248
E 0.250 6.350
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 0.290 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 10.08
Y 0.404 10.26
Z 0.413 10.49