When I first started mailling and looking around for ring sizes a LOT of them were in inches....it did my poor mind in trying to work out a conversion. I found a table similar to this one to be extremely helpful, and still refer to mine when it's not a size I am used to.
Have you got a ring ID that isn’t listed, here is how you can convert it yourself.
You need to convert the fraction to a decimal. You can do this by dividing the top number (numerator) by the bottom number