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.