Wallpaper Calculator
Rolls, drops and pattern waste — counted the way a decorator actually counts them.
Use the vertical repeat printed on the roll label: inches if you picked feet, centimetres if you picked metres. Every repeat is added to each drop as waste.
Order every roll in one go and check the batch (dye lot) number matches on all of them. One spare roll costs far less than a second delivery in a slightly different shade.
Drop method: each strip is cut to the wall height plus 4 in of trim, then rounded up to the next full pattern repeat. Openings only credit half their width, because the pattern still has to run above and below them.
How much wallpaper do I need? Count drops, not square feet
Most online estimators divide your wall area by the area of a roll and round up. That method quietly ignores the two things that actually decide how many rolls leave the shop: you cannot buy half a strip, and every offcut shorter than a full wall height is scrap. A roll of standard American wallpaper is 20.5 in wide and 33 ft long — about 56 sq ft on paper. In an 8 ft room you get three usable drops from it and throw away the last 8 ft, so the real coverage is closer to 41 sq ft. That is a 27% gap between the label and reality, and it is why people run one roll short.
This calculator works the way a decorator does. It cuts the wall into vertical drops the width of the roll, sets each drop to the wall height plus 4 in of trimming allowance, rounds that length up to a whole pattern repeat when the paper has one, and then asks how many whole drops fit in a roll.
Worked example: a 40 ft room with a 27 in repeat
Take a bedroom whose four walls add up to 40 ft, with a 9 ft ceiling, one door and two windows. Using 20.5 in x 33 ft rolls:
Drops needed: 40 ft is 480 in, divided by 20.5 in gives 23.4, so 24 drops. The door and windows credit half their width — roughly one drop in total — leaving 23.
Plain paper, no repeat: each drop is 108 in + 4 in trim = 112 in. A 33 ft roll is 396 in, so 396 / 112 = 3.5, which is three whole drops per roll. 23 drops / 3 = 7.7, so 8 rolls.
Same room, 27 in pattern repeat: each drop must now be cut to a whole number of repeats, so 112 in rounds up to 135 in (five repeats). Now 396 / 135 = 2.9, which is only two drops per roll. 23 drops / 2 = 11.5, so 12 rolls.
Same room, same paper width, 50% more rolls — purely because of the repeat. Nothing in the square-footage method warns you about that cliff edge, because the arithmetic is entirely about how a drop length divides into a roll length. If your ceiling had been 8 ft in that example, the repeat would have cost you nothing at all. It is worth running both numbers before you fall in love with a large-scale pattern.
The dye lot problem, in one paragraph
Wallpaper is printed in batches and each batch mixes ink fractionally differently. The batch number, or dye lot, is printed on the label. A homeowner who buys eight rolls, damages one during hanging and orders a ninth the following week will very often get a different lot — and the difference that was invisible in the shop reads as a pale stripe down one corner of the finished wall, in daylight, forever. Buy every roll in one order, check the lot numbers match before you cut anything, and keep one full spare roll in the loft for future repairs.
Accent walls and other short jobs
For a single feature wall, enter that wall's width alone. A 12 ft wide, 9 ft tall accent wall needs ceil(144 / 20.5) = 8 drops; at three drops per roll that is 3 rolls. The rounding hurts more on small jobs, since one wasted drop is a bigger share of the total — a 4 ft chimney breast still needs 3 drops and therefore a whole roll.
If the room has a picture window, sliding doors or built-in wardrobes that reach the ceiling, subtract their full width from the total wall width before you enter it, and leave the door and window counts for normal openings only. The half-width credit exists because a strip crossing a normal window still has to be cut full height so the pattern continues above and below it; only a floor-to-ceiling gap removes drops outright.
What this calculator does not model
It assumes rectangular walls of one height, no sloping ceilings or dormers, no border strips, and a straight or half-drop match rather than a random/free match hung deliberately at alternating angles. Very tall stairwells often need custom-length rolls. Thick vinyls and grasscloth may need seam overlap that eats another inch per drop, and paste-the-wall products with no repeat can sometimes be reverse-hung to reduce waste further. Add half a roll for a first-time job, a full roll if you are working around an awkward corner, and take a photo of the label so you can match the lot later.
Sources & further reading
Frequently asked questions
Why does the dye lot matter so much?
Wallpaper is printed in batches, and each batch — the dye lot or batch number — mixes ink very slightly differently. Two rolls from different lots can look identical in the shop and still show a visible colour band on the wall. Buy every roll for a room in one order, check the numbers match on the labels, and keep one spare so a repair years later still comes from the same run.
What is the difference between a straight match and a drop match?
On a straight match the design lines up at the same height on the next strip, so every drop is cut at the same point and waste is one repeat at most. On a half-drop match the design steps down by half a repeat, so alternate strips start at different points and offcuts pile up faster. This calculator rounds every drop up to a whole repeat, which covers a straight match exactly and a drop match with a little room to spare.
Why do doors and windows barely reduce the number of rolls?
The pattern has to run continuously above and below an opening, so a strip crossing a window is still cut to full height — only the middle piece is discarded, and it rarely fits anywhere else. Decorators traditionally deduct half of each opening's width and no more. If a wall has a picture window or double doors that reach the ceiling, subtract that width from the total before you enter it.
Does this work for peel-and-stick wallpaper?
The drop maths is identical, but peel-and-stick rolls are usually shorter — often 16 to 18 ft — and sometimes narrower, so enter the exact roll size from the product page instead of the presets. Removable panels are also less forgiving of trimming errors because you cannot slide them far once they grip. On a first attempt add a whole spare roll rather than the usual half.