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. Dividing by area under-buys by about a third, because it assumes the tail of each roll can be used somewhere and it cannot. That is why this page counts strips instead.

The pattern repeat is the whole game

The repeat is the input easiest to leave out, and leaving it out is the failure that ends with 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 — which is why this page asks for the repeat rather than assuming it.

What a half-drop actually costs

The obvious way to handle a half-drop match is to add half a repeat to every cut. That is wrong, and it over-orders badly. Each strip is still cut to the same length — up to the next whole repeat — because the pattern has to land in the same place on every strip.

What a half-drop costs you is between the strips. Come off a cut and the roll sits at pattern position zero; the next strip has to begin half a repeat further along, so that half repeat is scrap. The strip after it has to come back, and scraps another half. So n strips take n cuts plus n−1 half-repeats, and only the first strip off a fresh roll is free.

On a 6 ft feature wall with an 8 ft ceiling, a 20.5 in roll, a 25 in repeat and 4 in trim, that is 4 strips cut at 100 in. A 220 in roll fits two of them (100 + 12.5 + 100 = 212.5), so you need 2 rolls. Add half a repeat to every cut instead and each cut becomes 125 in, one strip per roll, and you buy 4 — twice what the wall takes, in a paper you cannot return once the wrapper is off.

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.

Where the strip count stops being exact

The room is treated as one continuous band of paper: the perimeter divided by the roll width, rounded up once. That is not how a room gets papered. An inside corner cannot be turned with a whole strip — the paper is cut there, a fresh plumb line is struck on the next wall, and the count rounds up on that wall, by itself. Papered wall by wall, the 12 × 12 room above wants 8 strips a wall and 32 in total against the 29 this page returns. The allowance usually swallows that on a plain square room. On one with more corners than walls — a chimney breast, a bay, a boxed-in duct — it will not, and every return adds another rounding.

The repeat is rounded the same way: once, for the whole job, at one ceiling height. A stairwell, a sloped ceiling or a dropped soffit each need their own run of the page.

Openings are not deducted at all. The perimeter runs straight through the door and the window. That over-orders rather than under-orders, which is the right way round, because the short pieces above and below an opening come out of offcuts this model does not track. On a room that is mostly glass the answer will be visibly high, and it is the one case worth taking the opening widths off by hand before you enter the length.

A missing dimension returns nothing rather than a guess. Leave a room dimension at zero and the perimeter would otherwise run round two walls of a room with no depth; leave the ceiling height at zero and a strip is nothing but its 4 in trim allowance, 99 of which fit a roll. Both come back empty instead. A feature wall is the exception the page does allow: on that setting only the first measurement is used, and the room width is ignored.

A roll shorter than one strip has no answer at all. The roll count is strips divided by the strips a roll yields, and if the cut comes out longer than the roll — a stairwell, or a short remnant entered by hand — no roll yields a single strip and there is nothing left to divide by. The page falls back to one roll per strip, which is not an order to place. A strip has to reach floor to ceiling in one piece, so the answer there is a longer roll, not more of this one.

Two smaller assumptions. The trim allowance is a flat 4 in on every strip — Brewster's figure, written for a ceiling that is level. And the strips are taken to come off the rolls in order, which is what the half-drop arithmetic above rests on; cut all your strips first, from several rolls at once, and the realignment loss falls somewhere else.