You are mixing measurements and definitions so no wonder you find it confusing. To simplify things take one overall measurement first e.g LENGTH
1. If a room is 12 feet long and a floorboard is 3 feet in length then you would need 4 boards to cover the length of the room(12 divided by 3 = 4).
2. Next WIDTH. If the room is 9 feet wide then for every foot width you would need 3 boards(4inches =1/3foot) thus for 9 feet you would need 9x3 boards(27).
3. The total therefore would be 27(across) x 4 long = 108 boards.
However in simple terms your total square footage is simply 12 x 9 which also equals 108 (sq.ft)(coincidence)
However, most DIY shops have have ready printed leaflets on calculating this sort of thing.
Hope this helps but if you give me a basic overall LENGTH by overall WIDTH I can do it for you.