There are three systems for designating drill diameters. Small drills use a numerical gauge (larger numbers correspond to smaller diameters). Larger drills use letters (“A” corresponding to the smallest diameter, “Z” to the largest). Drill diameters specified as a fraction of an inch are also common.

Drill #	Diameter (inch)	Diameter (mm)
80	0.0135	0.34
79	0.0145	0.37
78	0.016	0.41
77	0.018	0.46
76	0.020	0.51
75	0.021	0.53
74	0.0225	0.57
73	0.024	0.61
72	0.025	0.64
71	0.026	0.66
70	0.028	0.71
69	0.029	0.74
68	0.031	0.79
67	0.032	0.81
66	0.033	0.84
65	0.035	0.89
64	0.036	0.91
63	0.037	0.94
62	0.038	0.97
61	0.039	0.99
60	0.040	1.02
59	0.041	1.04
58	0.042	1.07
57	0.043	1.09
56	0.0465	1.18
55	0.052	1.32
54	0.055	1.40
53	0.0595	1.51
52	0.0635	1.61
51	0.067	1.70
50	0.070	1.78
49	0.073	1.85
48	0.076	1.93
47	0.0785	1.99
46	0.081	2.06
45	0.082	2.08
44	0.086	2.18
43	0.089	2.26
42	0.0935	2.37
41	0.096	2.44
40	0.098	2.49
39	0.0995	2.53
38	0.1015	2.58
37	0.104	2.64
36	0.1065	2.71
35	0.11	2.79
34	0.111	2.82
33	0.113	2.87
32	0.116	2.95
31	0.120	3.05
30	0.1285	3.26
29	0.136	3.45
28	0.14	3.56
27	0.144	3.66
26	0.147	3.73
25	0.150	3.81
24	0.152	3.86
23	0.154	3.91
22	0.157	3.99
21	0.159	4.04
20	0.161	4.09
19	0.166	4.22
18	0.170	4.32
17	0.173	4.39
16	0.177	4.50
15	0.18	4.57
14	0.182	4.62
13	0.185	4.70
12	0.189	4.80
11	0.191	4.85
10	0.193	4.90
9	0.196	4.98
8	0.199	5.05
7	0.201	5.11
6	0.204	5.18
5	0.205	5.21
4	0.209	5.31
3	0.213	5.41
2	0.221	5.61
1	0.228	5.79

Letter drill	Diameter (inch)	Diameter (mm)
A	0.234	5.94
B	0.238	6.05
C	0.242	6.15
D	0.246	6.25
E	0.250	6.35
F	0.257	6.53
G	0.261	6.63
H	0.266	6.76
I	0.272	6.91
J	0.277	7.04
K	0.281	7.14
L	0.29	7.37
M	0.295	7.49
N	0.302	7.67
O	0.316	8.03
P	0.323	8.20
Q	0.332	8.43
R	0.339	8.61
S	0.348	8.84
T	0.358	9.09
U	0.368	9.35
V	0.377	9.58
W	0.386	9.80
X	0.397	10.08
Y	0.404	10.26
Z	0.413	10.49

Source: https://littlemachineshop.com/reference/numberdrillsize.php and https://littlemachineshop.com/reference/letterdrillsize.php.

Even smaller drills do exist. See, for instance the table at https://www.custompartnet.com/drill-size-chart.