Well, usually. It will end up being one of 12", 16", possibly 19.2",
and rarely, 24". If you use either of the latter two, you end up
having to be careful about what you use for a subfloor.
Look up "span tables", online.
I submit that the way this question was asked implies that you're
awfully far behind the curve for what you need to know before
tackling a DIY building project of this magnitude.
I think you should spend a couple days with a book like
"wood frame house construction" by L. O. Anderson, William Oberschulte
You mean JOISTS. Most are 16" centers (leaving about 14 1/2" between) but
it is possible to do other methods in certain cases. The only time I've
heard 12" OC is for a deck using the Trex or other hybrid materials.
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