There is a simple trick to finding your right ring size. Just cut a small strip of paper and wrap it around your finger. Make sure that the paper strip is as tight as the ring should be. Now you mark the place where the paper overlaps. Then use a ruler to measure how many millimeters the paper strip is up to the mark. In the overview you will then find your perfect ring size:
Circumference 49.3 mm = diameter 15.7 mm = ring size 5 (50)
Circumference 51.8 mm = diameter 16.5 mm = ring size 6 (53)
Circumference 54.6 mm = diameter 17.4 mm = ring size 7 (55)
Circumference 57.1 mm = diameter 18.2 mm = ring size 8 (58)
Circumference 59.6 mm = diameter 19.0 mm = ring size 9 (60)