Sizing your finger for a ceramic Coral ring

Sizing  for a ceramic ring without trying it on can be a bit tricky, they are often not 100% round.  
If you don’t know your size, one of the best ways is to get an existing ring that fits your finger well, and measure the inside diameter. (Across the opening, from inside edge to inside edge) then either let me know the measurements, or convert them using the chart to a letter size :) 
Australian = British