Wallpaper calculator
A plain or random-match paper wastes only the trim, so 10% is plenty. A straight-match paper with a 32 cm repeat means every drop starts at the same point in the pattern, and the offcut between drops is dead. On a 2.4 m wall that can be a third of a metre lost per drop — 20 to 25% waste, and no calculator can shave it.
A standard European roll is 10.05 m long and 0.53 m wide, covering about 5.3 m². Divide the wall area by that, then add 10–25% depending on the pattern repeat: a plain paper wastes only the trim, a large straight-match repeat wastes far more.
How to count wallpaper rolls
Do not deduct doors and windows unless they are unusually large. A standard door removes about 1.8 m² but it interrupts two drops, and the offcuts above and below it are rarely long enough to reuse. Deducting the area while the wastage stays the same is how people end up a roll short. The exception is a room that is mostly glass — a wall of patio doors genuinely is not being papered and should come off.
Questions
A standard European roll of 10.05 × 0.53 m covers 5.33 m². American rolls are usually 8.2 m by 0.686 m, about 5.6 m².