1. This is a chart that will be extremely helpful for you to determine the ring size that you need. You can either make use of a tape or a small piece of string to measure the area that will be occupied by the ring. When it becomes a complete circle, make a mark on such string.
This will be helpful for you to compare with the chart that is mentioned below.
2. Here is another chart that can also be considered in determining the appropriate ring size. It measures inside the diameter of such ring in millimeters. This is normally used for double checking your results from the usual paper method. Apart from this, you can also make use of the ring that you wear daily and cross check the same with below chart.