Wallpaper calculator

Wallpaper is not sold by the square foot

Every strip has to run floor to ceiling in one piece, so the only question that matters is how many full-height strips you can get out of a roll. Everything left over at the end of the roll is scrap.

A 12 × 12 room has 48 ft of wall. At the standard US roll width of 20.5 in that is 48 × 12 ÷ 20.5 = 29 strips. With an 8 ft ceiling and Brewster's specified 4 in trim allowance, each strip is cut at 100 in. A 33 ft roll is 396 in, so it yields 3 strips and 96 in of offcut you cannot use.

Divide the same room by area and you get 384 sq ft ÷ 56.4 sq ft per roll = 7 rolls. The real answer is 10 before any waste allowance. Area-based wallpaper calculators under-buy by about a third, because they assume the tail of each roll can be used somewhere and it cannot.

The pattern repeat is the whole game

Almost no wallpaper calculator asks for the repeat, and this is the failure that leaves people one strip short of a finished wall.

Every strip must start at the same point in the design, so a cut can only be a whole number of repeats long. Take a 25 in repeat and a 102 in wall: the bare cut is 106 in with trim, but 106 is not a multiple of 25, so you cut at 125 in and bin 19 inches off every single strip. That is 18% of the paper gone before you have hung anything, and it is invisible in any square-footage calculation.

The effect is not gradual, it is a cliff, because it changes how many strips come off a roll:

• 8 ft ceiling, no repeat — cut 100 in, 3 strips per roll, 10 rolls for a 12 × 12 room.
• 9 ft ceiling, no repeat — cut 112 in, still 3 strips per roll, still 10 rolls.
• 9 ft ceiling, 36 in repeat — cut 144 in, only 2 strips per roll, and the same room needs 15.

A 50% jump in cost, from a number printed on the label that most calculators never ask for. On a half-drop match it is worse again: alternate strips start half a repeat further down the roll, so add half a repeat to every cut.

Order it all at once, and order it in double rolls

Wallpaper is printed in dye lots and the lots do not match. There is no equivalent of touching up paint — a roll bought later will read as a different colour on the wall, and you cannot re-paper one strip. Buy the spare with the order, and check every roll carries the same run or batch number before you cut anything.

The single/double roll naming is a pricing relic. The physical roll you receive is the double roll — about 20.5 in × 33 ft, or 56 sq ft — even though it is priced per single roll. Order by the roll you are actually handed and you will not end up with half of what you needed.

Finally: check the drop on the first strip, not the third. Hang it, step back, and confirm the pattern is centred where the eye goes — the middle of a chimney breast or a feature wall — before you commit to the rest.