Simply put your engagement ring on a ruler (as in the image) and measure its inner diameter.

The millimetric value you find is your ring size.

RİNG SİZE	RİNG DİAMETER
6	14,64 mm
7	14,96 mm
8	15,28 mm
9	15,60 mm
10	15,92 mm
11	16,24 mm
12	16,56 mm
13	16,87 mm
14	17,19 mm
15	17,51 mm
16	17,83 mm
17	18,15 mm
18	18,47 mm
19	18,78 mm
20	19,10 mm
21	19,42 mm
22	19,74 mm