Unfortunately, I just found out that the ring mandrel that I got does not follow the U.S standard nor UK standard. But fret not. Here's how you can measure your ring size :
1. Simply measure the circumference (distance around) of your finger (with something flexible, such as a strip of paper or piece of string) and then measure the paper/strip against a ruler to get the circumference of your ring size in mm/cm.
2. Or, measure the inner diameter (distance across the inner ring) in millimetre / centimetre of a ring you currently wear. Then multily the diameter with 3.14 to get the circumference of your ring in mm/cm.
(Photo and guideline from here)
I've also added the link to this post in my sidebar