In the industry, things are held to general standards and not usually to exact standards.
In muzzleloading arms, a 20 gage is a .62
A 24 gage is a .58
A 28 gage is a .54
A 12 gage is a .72
A 16 gage is a .66
A 10 gage is a .78

However, below is a chart of "true" sizes.

Guage Caliber
1 1.669
2 1.325
3 1.157
4 1.052
5 .955
6 .919
7 .873
8 .835
9 .803
10 .775
11 .751
12 .729
13 .710
14 .693
15 .677
16 .662
17 .649
18 .637
19 .629
20 .615
21 .605
22 .596
23 .587
24 .579
25 .571
26 .563
27 .556
28 .550
29 .543
30 .537