This Blender add-on adds 3D printable precisely sized rings to the stage. It contains 80 official ring sizes. The buttons on hover shows national sizing information.
Open User Preferences (File menu or ctrl+alt+u). On the Add-ons tab press "Install from file" button at the bottom of the window. Select the zip or py file. If it doesn't show search for "Ring". Tick the add-on then press "Save user settings" button at the bottom of the window.
In object mode press one of the 80 size buttons located in Create panel to add a 3D pint ready ring. It will be very small compared to Blender's normal sizes. One Blender unit is 1 meter. On default you see meters. When you add the ring zoom in to see the small real life size (centimeters) ring. Notice that the ring have simple geometry with two modifiers: Solidify and Bevel.
You can download it from here, or use the source code I provided at the bottom.
As you can see it is very primitive, and long. This is the first time I created add-on for Blender or used Python, also I needed this add-on fast. So it shows. I am aware this could be much better. :) Only thing I want to tell you about it: I didn't type out most of it. I have better thing to do :D I wrote a PHP script that created the python script for me based on ring size data. As you can see the license let you do whatever you want with it. So if you make a better add-on from it: let me know.