Ring size in numbers?

Hi everyone a customer has sent me their size for a ring , the ring sizer they got was numbers not letters . ( is that American? )
It is a size 18 has anyone any idea what letter that would convert to in the Uk , am I right in thinking it’s a size S?
Thank you in advance.Attachment 12900

No it's not American. It could be the internal diameter in mm, or something quite other.

Check it out with something that can be sent and verified with your own ring stick, such as Cooksons https://www.cooksongold.com/Jeweller...zer&channel=uk Dennis.

Thanks Dennis, the sizer they used is on its way . Before starting a gold ring I always send a couple of silver ones for people to wear as more times than not the size changes to what they thought they wanted .