If you don't know your ring size, follow these instructions and you will be able to determine it.
#1 - Cut a strip of paper no wider than 1/4".
#2 - Wrap the paper snugly (as you want the ring to fit) around your finger...mark the paper where it meets.
#3 - Measure the marked length of paper and compare it to the chart below.
Note: If your knuckle is significantly larger than the base of your finger, you should measure the knuckle as described above and pick a size somewhere in between the two.