To measure the diameter (width of your ring on the inner side), you will need a ring that fits your relevant finger well and a measuring tool.
Choose a ring you already wear comfortably on your desired finger.
Now measure from side-to-side of the innermost part of the ring.
View the corresponding measurement against the ring chart, and Voilà this is your size!
To measure the circumference (circle around the base of your finger), you will need a piece of string, a pen and a measuring tool.
Place the string around your finger. Make sure that it is wrapped comfortably and not too tight. Then mark the spot where the string meets – this will be the length of the string.
Place the string on your measuring tool / ruler and measure in millimeters. This is your ring circumference.
Finally, check the measurement chart to view your ring size!