HOW TO TAKE THE MEASUREMENT FOR A RING
You can buy a ring sizer, easy to find and really cheap.
It is used as a belt on the finger that you have to tighten until the ring sizer struggles to get out. At that point, read the size indicated and select it at the time of purchase.
Jeweler Near You
If you don't want to buy the ring sizer you can go to any jeweler and ask them to take the size of your finger: it will be in Italian measurements but by comparing it with the diagram below you can easily trace the American one that I use on my site
E.g. Italian size 17 ----------> American 8
Or take a ring that fits your predestined finger and measure its INTERNAL diameter, as shown in the drawing below, trying to be as precise as possible (in ring sizes 1 mm makes the difference!)
Ex. fit inner diameter 18mm -----> US 8
Most Common US Sizes
4 misura EXTRA SMALL
5 e 6 misure SMALL
7 misura MEDIUM
8 e 9 misure LARGE
10 misura EXTRA LARGE