To make the rings I reference the Italian chart, this is probably different if you are from another country. Below on the Ring Size Chart you can compare the values of your inner diameter to the Italian, British, American and Japanese sizes.
If you don’t know your size this is a little trick to measure your inner diameter. Take a piece of string and wrap it around the interested finger. Mark the point of intersection. Check the length on a ruler. This will give you the circumference. Remember to calculate it in mm. Divide the value of the circumference by 3.14, this will give you the diameter. If your ring falls between two sizes, I recommend you order the larger size.
The best way to find out your exact size is to measure it with a ring sizer. If you don’t own one is always best to ask your local jeweller.