We use US ring sizing for all of our rings and will always provide the inner diameter in millimetres to accompany it. 

The below chart shows the US ring sizing and the correlating measurements of the diameter of the inner circle of the ring in millimetres.

Larimar Agate Pendant

 Measure the inner diameter of an existing ring with a ruler as shown on the picture if you are uncertain of your size:

Larimar Agate Pendant