Buying wallpaper by area alone almost always comes up short, because the leftover strip below the last full-height drop on each roll is wasted. This calculator uses the drop-based method decorators actually use: it works out the cut length of each drop, how many whole drops you can get from one roll, and how many drops it takes to go around the room.
How it works
drop length = wall height + trim allowance
(rounded up to a whole pattern repeat when a repeat is set)
drops per roll = floor(roll length / drop length)
drops needed = ceil(room perimeter / roll width)
rolls = ceil(drops needed / drops per roll)Rounding the drop length up to a whole pattern repeat is what makes patterned papers use more rolls than plain ones — every drop has to start at the same point in the design so the pattern matches across seams.
Worked example
A room with a 15 m perimeter, 2.4 m walls, a standard 10.05 m × 0.53 m roll, a 0.5 m pattern repeat and a 0.1 m trim allowance:
- Raw drop = 2.4 + 0.1 = 2.5 m, rounded up to 2.5 m (5 × 0.5 m repeat)
- Drops per roll = floor(10.05 / 2.5) = 4
- Drops needed = ceil(15 / 0.53) = 29
- Rolls = ceil(29 / 4) = 8 rolls
What to watch
Doors and windows are ignored by this estimate, which is deliberate — the offcuts above and below openings are rarely reusable, so counting full drops keeps you from running short. On a room with a very large opening you can subtract a drop or two.
Batch numbers matter more than most people expect. Always order all your rolls at once and keep a spare from the same batch for future repairs.