The easiest way to measure the size is to measure the internal diameter of one of your rings that fits your finger comfortably.
The measurement match the inside edge of the ring. This goal is not entirely accurate, but gives you a good idea of ​​what size you need.

Diameter:        Circumferense:     Ring size:

14,1 mm          44,2 mm                  3 

14,5 mm          45,5 mm                  3,½
14,9 mm          46,8 mm                  4
15,3 mm          48,0 mm                  4½
15,7 mm          49,3 mm                  5
16,1 mm          50,6 mm                  5½
16,5 mm          51,9 mm                  6
16,9 mm          53,1 mm                  6½
17,3 mm          54,4 mm                  7
17,7 mm          55,7 mm                  7½
18,1 mm          57,0 mm                  8
18,5 mm          58,3 mm                  8½
19,0 mm          59,5 mm                  9
19,4 mm          60,8 mm                  9½
19,8 mm          62,1 mm                10
20,2 mm          63,4 mm                10½
20,6 mm          64,6 mm                11
21,0 mm          65,9 mm                11½
21,4 mm          67,2 mm                12
21,8 mm          68,5 mm                12½
22,2 mm          69,7 mm                13
22,6 mm          71,0 mm                13½