Cut a length of string approximately 6 inches long. This cannot be a string with any stretch. If such string is not available, a piece of paper about 6 inches long and 1/4 inch wide also works.
Have a friend help if necessary. Wrap the string or strip of paper around the ring finger, pulling it snug but not too tight. Mark the spot where the end of the string or paper overlaps.
Take the string or piece of paper from around the finger. Hold it up to a ring measuring chart, or use the following measurements: size 5 is 1 1/16 inch, size 6 is 1 1/8 inch, size 7 is 1 3/16 inch, size 8 is 1 1/4 inch, size 9 is 1 5/16 inch, size 10 is 1 6/16 inch, size 11 is 1 7/16 inch, size 12 is 1 1/2 inch, size 13 is 1 9/16 inch, and size 14 is 1 5/8 inch. Men's and women's rings are measured identically.
If the string method falls between two sizes, choose the larger size for the ring.