How to Use This Tool
Enter the room and, most importantly, the pattern repeat printed on the roll label. That one number moves the answer more than the room size does.
Paper is sold by the roll and hung in drops
A roll is a fixed length of a fixed width. What you get out of it is a whole number of full-height drops, and whatever is left at the end of the roll is too short to be another one. Calculating in square feet hides that rounding completely, which is why area-based answers are consistently short.
96 in wall, 21 in repeat, 33 ft roll, 20.5 in wide cut length ceil(96 / 21) = 5 repeats = 105 in waste 105 - 96 = 9 in per drop drops per roll plain 396 / 96 = 4 repeat 396 / 105 = 3 12 x 14 room, 52 ft perimeter drops 624 / 20.5 = 31 plain 31 / 4 = 8 rolls repeat 31 / 3 = 11 rolls
Why the repeat forces a longer cut
On a patterned paper every drop has to begin at the same point in the design, or the pattern will not line up across the seam. So each drop is cut to a whole number of repeats, and the offcut at the bottom is scrap unless it happens to be long enough to use above a door.
A half-drop match is worse again: alternate drops start half a repeat down the roll, so the effective repeat for cutting purposes is longer and the waste per drop grows. A free match paper, with no pattern to align, has none of this.
The trim allowance can hide the whole effect
Most installers add two to four inches per drop for trimming top and bottom. That allowance is real, but it changes where the drop length lands relative to the roll length, and it can make the repeat look free. At 96 in with a 4 in trim the cut becomes 100 in plain and 105 in patterned, and a 33 ft roll yields three drops either way — so the repeat appears to cost nothing.
It has not stopped costing anything; the plain paper has simply started wasting as well. The lesson is that drops per roll is a step function, and small changes in cut length either cross a step or do not. Set the trim you will actually use and read the drops-per-roll figure, rather than assuming the repeat always adds rolls or never does.
Do not deduct doors and windows the way you would with paint
With paint you subtract the opening area. With wallpaper you usually cannot, because the drop beside a window still has to run full height, and the short pieces above and below it come from the same roll length. A door can save part of a drop, a small window rarely saves anything, and treating openings as area removed is how people end up a roll short.
Buy one extra roll, and check the batch number
Every roll should carry the same batch, shade or dye-lot number. Rolls printed in different runs differ enough in colour to show across a seam, and there is no fixing it later. Buying the spare roll at the same time as the rest costs one roll; buying it a year later usually means it does not match.
What is not counted here
Ceilings, feature walls with a centred motif, and any wall where you want the pattern to break at a specific height all need setting out by hand. Paste, lining paper and trimming tools are separate.
