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/paper/ thread to measure the area that will be occupied by the ring. When it becomes a complete circle, make a mark on such string/paper/ thread. This will be helpful for you to compare with the chart that is mentioned below. For extra width, you can add another size to accommodate.
1. Cut a thin strip of paper/ thread.
2. Wrap the paper around your finger. Make sure the paper is below the joint of your finger and close to your knuckle.
3. Mark the spot where the paper/thread meets and measure the distance with your ruler.
4. Use the following chart to determine your ring size. Rings also come in half sizes, the measurements of which, of course, will fall somewhere in between the whole sizes