Floors and walls
Coverings that go down or up: flooring, tiles and plasterboard.
What these have in common
These are all sold in packs of a fixed size and cut to fit, so the answer is always an area divided by a pack, rounded up, with something added for the offcuts you cannot use.
How much to add for waste is the judgement in all of them, and it depends on the shape of the room and the pattern you are laying. Every calculator here lets you set it and says what it assumes.