I use a US-sized ring mandrel to measure all my rings, so sizes may be approximate since I do not measure by diameter or circumferences.
The chart shows a standard circumference & diameters in millimeters. Please measure the inner rim of the ring you are currently wearing. If your size falls in between measurements, choose the larger size.
Remember that your ring measurement should slide over your knuckle comfortably.
Measure your finger a few times, finger changes size at different times of the day.
If you have an asian-sized ring, you can use the chart below to convert to US size.
Rings may have a slight variation in standard sizes depending on manufacturer & country of origin.