The sizes shown in the shop equal the circumference of the ring in millimeter.

1. Take a strip of paper, rap it around the inside of a ring you already own or the desired finger and make sure it fits over the knuckle, mark the paper where it overlaps and messure the marked length. 

This way you get the inner circumference (mm) wich equals the size shown in the shop and chart.

2. Take a ring you already own, that fits you on the desired finger, put it on a ruler and messure the inner diameter (mm) and compare it to the chart.