I did this for my products descriptions, use it if you like...
Size 5
Inside diameter: 0.618 inches - 15.7 millimeters
Inside circumference 1.94 inches - 49.3 millimeters
Size 6
Inside diameter: 0.65 inches - 16.51 millimeters
Inside circumference: 2.04 inches - 51.9 millimeters
Size 7
Inside diameter: 0.682 inches - 17.32 millimeters
Inside circumference: 2.14 inches - 54.4 millimeters
Size 8
Inside diameter: 0.714 inches - 18.14 millimeters
Inside circumference: 2.24 inches - 57 millimeters
Size 9
Inside diameter: 0.746 inches - 18.95 millimeters
Inside circumference: 2.34 inches - 59.5 millimeters
Size 10
Inside diameter: 0.778 inches - 19.76 millimeters
Inside circumference: 2.44 inches - 62.1 millimeters
Size 11
Inside diameter: 0.81 inches - 20.57 millimeters
Inside circumference: 2.54 inches - 64.6 millimeters
Size 12
Inside diameter: 0.842 inches - 21.39 millimeters
Inside circumference: 2.65 inches - 67.2 millimeters
Source:
http://en.wikipedia.org/wiki/Ring_size
Cheers!