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I'm not sure I understand what you mean. Are you trying to figure out the actual diameter of a ring from a photo of that ring? I'm not sure that could be done without any real-world scale references in the picture to compare it with.
There are conversion charts that will give you the US/Canadian ring size based on the diameter/circumference of the ring in mm, but guessing the actual scale of the ring based on a photo isn't going to be accurate unless there are other objects in the photo that we know the sizes of and can extract a comparison measurement by distance, etc.
Forgive me if I'm wrong in my assumption of what you are asking.
