Whichever method you choose, here are some quick tips to get a reliable result:
Measure your finger preferably at the end of the day when your hands will be a little inflated by the events of the day.
Measure the widest part of your finger.
Prefer the bigger if you hesitate between two sizes.
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. For extra width, you can add another size to accommodate.