Ring size is determined by inner diameter, not circumference. To find yours, cut a thin piece of paper and wrap it around your finger, keeping it close to your bottom knuckle. Mark where the paper meets and measure its length with a ruler. US 7 is the standard ring size. When no size is noted, the ring is a SIZE 7. Measurements below in millimeters.
SIZE 6: 16.4 mm inner diameter, 51.5 mm inner circumference
SIZE 7: 17.4 mm inner diameter, 54.5 mm inner circumferenceSIZE 8: 18.2 mm inner diameter, 57.0 mm inner circumference