Ring sizing systems throughout the world vary. It would be great if everyone used the same system, it would certainly make things easier for me, but we don't. Below I will list what I know of these systems and include a reference chart at the bottom.
I make rings in both the UK and US systems.
United Kingdom and Commonwealth.
In the UK we use a system referred to as the Wheatsheaf system (although few call it wheatsheaf). The scale goes from A to Z with half sizes in between . Beyond Z is often expressed as Z+ 1, 2, 3, 4 etc. where each number represents a half size. This system is also used by many Commonwealth countries including Australia, New Zealand and South Africa to name a few. The only obvious exception is Canada, which uses the US scale. Virtually everything I make is in the UK sizing scale.
This system was revised in 1987, but is virtually identical to the previous system. As the UK was entirely imperial measurements, it shouldn't come as a surprise that this scale was based on a imperial measurement. Each difference in size was based on an increase of 1/64th of an inch in diameter. Although the UK is now decimal/metric, this system is still used.
United States and Canada.
The US and Canada use a system of numbers from 0 to 13 ordinarily, but can be extended beyond this as the difference between full sizes is 4/125" (0.032 inches (0.81 mm) of internal diameter, a wider difference between sizes than the UK system, but then there are 1/4 sizes in between.
Europe uses 3 different systems which are all based on a metric measurement.