I would count on eight 2.5 inch squares from a 21 inch wide piece of fabric.
I would only count on eleven 2.5 inch strips from a 30 inch length of fabric.
The way I lay my ruler out and cut, I usually end up with my strips being 2-3 threads wider than the "target number" - so by the time I have cut several strips, my theoretical usage should have been something like 15 inches, but my actual usage would probably be closer to 15.5 inches.