As I research a set of new wheels, the wider wheels (~23mm) and wider tires(25mm) are very popular for training. Discussion generally centers around the quoted sizing, but what about the "as installed" sizing? I've come across some info citing that the installed width of tires on a wider rim exceeds the quoted size. For example, some 25mm tires installed on 23mm rims will measure out close to 28mm.

How do I calculate this prior to purchase? Is there a standard forumla?