A finger gauge is a device designed to determine the size of fingers, which helps with choosing the ring of the required size. In its simplest form, this function can be performed by ordinary plastic or paper tape with marked marks. It is wrapped around the finger and thus the size of the finger is known.
(Download Here) . The finger gauge is also a complex of many plastic or metal rings of various sizes. Each of them has a special marking. The most common is the standard scale, showing the size of the ring on the inside, expressed in millimeters. The ring size varies from 13 to 25 mm, increasing in increments of 0.2 or 0.5 mm.
Some countries use their own scales. For example, the diameter may be expressed in inches. In some cases, measuring the inner circumference of the ring is used.
In Canada and the USA, standard units of size are used, which start from zero to sixteen and increase at intervals of 1/4. In Australia and Ireland, letter designations are used - from A to Z. In China and Japan, a special numerical system has been adopted, the sizes of which vary from 1 to 27. There are special tables created to convert values from one scale to another
(Table here) .
More details about ring sizes -
Here