How to find your ring size
Cut a long strip of a paper, wrap it around the chosen finger, tightly, on the largest part of your finger, usually your knuckle.

Make a mark where it overlaps and measure it on a ruler, then compare it to the ring sizes below:
 

Ring Size millimetres   Ring Size millimetres
E 42   Q 57.5
F 44   R 59
G 45   S 60
H 46   T 61
I 47.5   U 62.5
J 49   V 63
K 50   W 65
L 51   X 66
M 52.5   Y 67.5
N 54   Z 68.5
O 55   Z1 70
P 56   Z2 71

Alternatively, you could always go to a jewellers and ask them to measure your finger.