Take a strip of paper and wrap it around the finger you would like to wear the ring on. Mark the spot where the end has come round fully. Then lay the strip against a ruler. You now have your circumference. Bear in mind, different fingers on the same and the opposite hand may have different sizes. Therefore, measure the specific finger on which you are planning to wear the ring. Also, measure your finger at the end of the day when your finger is likely to be largest.