Pattern repeat and the yardage math that decides how much you actually buy
Two rooms of the same size can need very different quantities. The variable is the repeat, and whether it is a straight or a half-drop match. A worked example follows.
The wall area is the starting point of a wallpaper estimate, not the answer. The number a buyer actually orders is driven by the pattern repeat and the match type, because those decide how much of every strip becomes waste at the ceiling line.
Repeat is the vertical distance before the pattern starts over. On a large repeat, each drop is cut up to the next full repeat above the required length so the pattern aligns with its neighbor, and the leftover is scrap. Work a real case: a wall 9 feet high (108 inches) papered in a pattern with a 25-inch straight-match repeat. The cut length is rounded up to a whole number of repeats above 108 inches — five repeats is 125 inches — so each 108-inch drop carries 17 inches of waste. A European double-roll bolt of about 33 usable feet (396 inches) yields three of those 125-inch drops, not the three-and-a-half a naive area calculation would suggest.
Match type compounds it. A straight match places the same point of the pattern at the same height on every strip, so waste is predictable. A half-drop match staggers the pattern down by half a repeat on alternate strips, which hides seams but raises waste because adjacent strips begin at different points. Random-match textures and many grasscloths carry no repeat waste at all, which is one reason they estimate tighter.
So the purchasing discipline is to demand the repeat and the match type on every quote and run the cut-length math before committing a quantity, rather than trusting a square-foot rule of thumb. A buyer who orders by area alone on a large half-drop repeat comes up short, and the reorder that follows carries a dye-lot risk the first order did not.