Wallpaper calculator

How the rolls are calculated

cut length = whole repeats × repeat (plus half a repeat with an offset match) · strips per roll = roll length ÷ cut length (rounded down) · strips = wall run ÷ roll width (rounded up) · rolls = strips ÷ strips per roll (rounded up)

Wallpaper is not bought by the square meter but by the roll, and a roll is cut into whole strips of the same length. What is left at the bottom of a roll is unusable unless another whole strip fits into it — which is why dividing the wall area by the roll area, the way most calculators do, under-orders on any paper with a repeat.

First the wall run is divided by the roll width and rounded up: that is how many strips have to be hung. Use the room perimeter if you are papering every wall, or one wall's length if you are papering one. For other jobs you can work the room area out in the area calculator.

Then it looks at how many whole strips come out of one roll: the roll length divided by the cut length of one strip, rounded down. Finally the strip count is divided by that and rounded up — those are your rolls. There is no separate waste percentage, because the waste is already inside the cut length; the calculator reports it as the real length it is, in metres.

Pattern repeat and offset match

The repeat is the distance over which the design repeats, and it is printed on the roll label. For the design to line up, every strip has to start at the same point in it, so a strip is cut not to the wall height but to the first whole number of repeats above it. A 2.6 m wall plus a 10 cm allowance is 270 cm, and on a 32 cm repeat the strip is cut at 9 × 32 = 288 cm. Those 18 cm are unavoidable.

An offset (half-drop) match means every second strip starts half a repeat lower, so half a repeat is added to every strip. On a 64 cm repeat a straight-match strip is 5 × 64 = 320 cm and an offset one is 5.5 × 64 = 352 cm — and that is the difference between three strips per roll and two, which for the same room is 9 rolls against 14. That is why the offset is its own checkbox rather than a detail.

Any saving from nesting one strip's offcut into the next is not counted. With an offset match the offcut can sometimes be used, but whether it can depends on the roll length, the drop and the hanging order, and we have nothing to back a figure with. So the answer errs a roll high rather than a roll short — the safer direction with wallpaper.

Doors, windows and full-height openings

Openings here are not deducted by area but by strips, and only when they run to the floor. If an opening reaches the wall height (within a 10 cm measuring tolerance), there is no wall there and so no strip: as many whole strips as fit across the opening's width are taken off.

Ordinary doors and windows deduct nothing. Paper still runs above a window and above a door head, and the strip is still cut from a full length — the piece left over above an opening can rarely be used elsewhere, because the pattern no longer falls in the same place. So a 2.05 m door in a 2.6 m room is still a full strip.

Every result lists the openings you entered and which of them counted as full-height and which did not, so you can see the rule applied to your own measurements instead of taking it on trust.

Worked example: a 4 × 3 m room

A 4 × 3 m room has a perimeter of 2 × (4 + 3) = 14 m, with walls 2.6 m high. The roll is 53 cm wide and 10.05 m long, the repeat is 32 cm and the trim allowance 10 cm. Each strip is cut at 288 cm, 3 whole strips come out of a roll, and 14 m of wall needs 1400 ÷ 53 = 26.42, so 27 strips. That is 27 ÷ 3 = 9 rolls with no spare strip, leaving about 12.69 m of unused paper.

If that room has a 2.4 × 2.6 m patio door, it runs to the floor: 240 ÷ 53 = 4.53, so 4 whole strips drop out. That leaves 23 strips, and 23 ÷ 3 = 7.67, so 8 rolls. A 0.9 × 2.05 m door and a 1.4 × 1.2 m window would change nothing — it would still be 9 rolls.

On top of the 9 rolls, buy one more from the SAME BATCH. Wallpaper shade varies from batch to batch, so a roll bought later often no longer matches the ones on the wall — and that is what makes it different from paint, which you can always top up. The batch (dye lot) number is printed on every roll.