Gauge/Inch to Millimeter Conversions are as follows (numbers in ( ) are standard conversion numbers) ::

20g = .80 mm
18g = 1.0 mm
16g = 1.3 mm
14g = 1.6 mm
12g = 2.1 mm (2 mm)
10g = 1.6 mm
8g = 3.2 mm (3 mm)
6g = 4.1 mm (4 mm)
4g = 5.2 mm (5 mm)
2g = 6.6 mm
0g = 8.3 mm (8 mm)
00g = 9.5 mm (10 mm) (may be considered 9mm by some manufacturers)

INCHES

1/4"=6.35~6mm

5/16"=7.9375~8mm

3/8"=9.525~10mm

7/16" = 11.125~11 mm

1/2" = 12.7~12 mm
9/16" = 14.2875 ~14 mm
5/8" = 15.875~16 mm

11/16"=17.4625~17mm 
3/4" = 19.05~ 19 mm

13/16"=20.6375~21mm 
7/8" = 22.225~22 mm

15/16"=23.1825~23mm 
1" = 25.4 ~25 mm

1 1/8"=28.575~29mm

1 1/4"=31.75~32mm

1 3/8"=34.925~35mm

1 1/2"=38.1~38mm

1 5/8"=41.275~41mm

1 3/4"=44.45~44mm

1 7/8"=47.625~48mm

2"=50.8~50mm 



Ball Conversions, Inch (fraction) to Inch (decimal) to Millimeter ::
3/32" = 0.094" = 2.4 mm
1/8" = 0.125" = 3.1 mm
5/32" = 0.156" = 4.0 mm
3/16" = 0.187" = 4.8 mm
1/4" = 0.250" = 6.3 mm
5/16" = 0.312" = 8.0 mm
3/8" = 0.375" = 9.5 mm
7/16" = 0.437" = 11.0 mm
1/2" = 0.500" = 12.7 mm
5/8" = 0.625" = 15.9 mm
3/4" = 0.750 = 19.0 mm