Know your Ring Size/How to I find out my ring size using UK method.
Rings are measured to the leading edge on a wheat sheaf ring stick.  So a size P would mean the edge of the ring just covers the letter P.

Ring Width and Finger Measurement
When sizing your fingers using ring gauges try and use a similar width gauge to that of the ring you are ordering. On narrower rings 4-5mm use a narrower gauge and for wider rings 7-10mm use a wider gauge. 
A general rule of thumb (no pun intended) if you don't have a wider ring gauge then go up one size, e.g. if your gauge is narrow and your finger size is an M then size N would be a better fit in most cases.

Here are some examples:
Example - normal finger size equals M
Ring Width 4-5mm ring (Thin) then no difference, so a size M
Ring Width 6 - 7 mm (Medium) then go up half a size to an M 1/2 
Ring Width 7mm - 10mm (Wide) then go up a full size to a size N
Note, all the above is for guidance only, people have different knuckle sizes and their own preference on how loose or tight they wish to wear rings on their fingers.