Welding Question

This is a slightly embarrassing question to ask, because my father made his living as a welder most of the time that I was growing up -- but I never actually learned much about it from him.  (It wasn't in a book from the library, so I wasn't much interested.)  When welding two pieces of steel or aluminum together in a butt weld, what additional thickness can one expect the weld to add?  Say you welded two pieces together that were 12" long each; how long would the final product be?  My recollection from looking at welds, and from reading the Wikipedia page, suggests that welding might add 1/8" of an inch to the length.