There are several factors that affect ring size besides actual finger measurement. First, fingers are not perfectly round, although most rings are. Second, the difference in size between finger joints and the place where a ring sits also affects the choice of ring size. Third, fingers change size in response to body chemistry and temperature, sometimes by several sizes. 

The size that is chosen is also affected by the specific ring. Wider rings and those in which a stone or setting projects toward the finger will require larger sizes than thinner rings, even for the same finger. Often, the best way to determine ring size is to go to a jeweller for a fitting.

Keeping this in mind, here is a method to help you determine your approximate ring size and a table showing the conversions between UK, US and foreign sizing.

DETERMINING YOUR RING SIZE

1. Cut a strip of paper about 5 inches long and about 1/4-inch wide. 
2. Wrap the paper strip around the widest part of the appropriate finger. Often, this will be at a joint, but it may be in a number of locations and may differ from hand to hand. You can use an old ring to help find the widest spot. 
3. Use a pen or pencil to mark the point on the paper where the ends of the paper strip overlap (form a complete circle). 
4. Use a ruler to measure the length from the outside end to the mark on the paper to the nearest 1/16" of an inch (decimal equivalents are given in the table) or millimeter, rounding up if necessary. 
5. Find the closest measurement on the size chart to determine your ring size. Use millimeters for a more accurate size.