Like most jewellers our silver rings are measured to the leading edge of the ring.
The leading edge of the ring is basically the bottom edge of the ring when it is placed on a jewellers measuring cone. This is particularly important where you are dealing with rings that are very wide and sometimes span over 2 or sometimes 3 sizes.
The sizes shown in the chart are based on UK and US ring sizes. There are of course other charts the most notable for Japan and Europe.
Ring sizes in the UK are shown by the letter, whilst the US sizes are shown in green in the chart below. In the photograph you can clearly see the difference between a large size Z and a relatively small size L. There are large sizes in the UK frequently known as Z+ i.e. Z+3 is 3 sizes above a size Z.
The chart shows the actual circumference (millimetres) of the ring shown in blue. (second column). Uk ring sizes are shown as a letter in the first column and US ring sizes are shown in the (right) third column.
Please bear in mind that when your fingers are hot they are going to be a little larger than when cold. In winter the same finger can sometimes be up to 2 sizes smaller than in the summer. So when ascertaining your size it is best to take a measurement when the finger is cold and another when the finger is hot, using the average of the two sizes for everyday use.
They measure a persons finger using a previously sized ring that fits. To measure a ring itself jewellers use a long cone shaped tool that is graded with a scale of the various sizes.