Wallpaper
Wallpaper roll maths: double rolls, repeats and drop match
Wallpaper is not bought by area. A room is a number of drops, each cut to the pattern, and the rolls are however many it takes to give you that many drops. Here is the arithmetic.
Why area does not work
Paint is bought by area because it spreads. Wallpaper does not spread; it is hung in full-height strips called drops, each the height of the wall plus a little to trim at top and bottom, and each cut so its pattern lines up with the drop beside it. Whatever is left at the end of a roll, if it is shorter than a drop, cannot be used on the wall. Dividing the wall area by the roll area therefore always underestimates, sometimes by several rolls. Professionals count drops.
The method
A door takes out a full drop width, because there is no wall beside or below it for the paper to cover, apart from a short piece above. A window takes out about half its width, because the paper still runs above and below it, and those short pieces use paper. These are the conventions the wallpaper roll calculator uses.
US double rolls and single rolls
US wallpaper is sold in double rolls, commonly 20.5 in wide and 33 ft long, about 56 sq ft. But it is usually priced per single roll, half a double roll. A paper labelled $45 is usually $45 per single roll and $90 per double roll you actually carry home. Wider US rolls, 27 in by 27 ft, are also common. European rolls are about 0.53 m wide and 10.05 m long, about 5.3 m², and are priced and sold as one roll.
| Roll | Width × length | Area | Priced per |
|---|---|---|---|
| US double roll, standard | 20.5 in × 33 ft | about 56 sq ft | Single roll (half) |
| US double roll, wide | 27 in × 27 ft | about 61 sq ft | Single roll (half) |
| European roll | 0.53 × 10.05 m | about 5.3 m² | Roll |
The sizes are on the label; the standard sizes reference lists them.
A worked room
A 12 × 10 ft bedroom with a 21 in repeat
8 ft walls, one door, two windows, US double rolls 20.5 in × 33 ft, straight match.
- Perimeter: 44 ft
- Width to cover: 44 − 3 − 2 × 1.5 = 38 ft
- Drops needed: 38 ft ÷ 20.5 in = 22.2, rounded up to 23
- Drop length: 96 + 4 = 100 in, rounded up to five 21 in repeats = 105 in
- Drops per roll: 396 in ÷ 105 in = 3.77, rounded down to 3
- Rolls: 23 ÷ 3 = 7.67, rounded up to 8 double rolls, 16 single rolls
- Paper left as roll ends: 24% of what you bought
From the wallpaper roll calculator. The 27 in wide rolls need only 17 drops for the same room, and six double rolls.
What the repeat does
The pattern repeat is the vertical distance before the pattern repeats itself. Every drop has to start at the same point in the pattern, so its length is rounded up to a whole number of repeats. Whether that costs extra rolls depends on how the drops fit the roll.
The same room with 7.5 ft walls
- No repeat: drop 94 in, 4 drops per roll, 6 double rolls, 9% roll-end waste
- 21 in repeat: drop rounded up to 105 in, 3 drops per roll, 8 double rolls, 24% waste
The repeat costs two double rolls here, a third more paper. With 8 ft walls, the drop without a repeat is already 100 in, too long for four drops per roll, so the repeat costs nothing extra. From the wallpaper roll calculator.
Large repeats, 25 in and more, are common on bold patterns and murals and can cost a lot of paper. Check the repeat on the label or the sample book before choosing a paper for a large room.
Straight match, drop match and random match
- Straight match: the pattern lines up horizontally across every drop. Each drop starts at the same point in the repeat.
- Drop match (half-drop): the pattern on each drop is offset by half a repeat from the one beside it. Alternate drops start half a repeat lower, which adds half a repeat to each drop’s length in practice. In the room above, the drop length becomes 115.5 in, still three drops per roll, so still eight rolls, but on other heights it costs more.
- Random match: no pattern to line up, such as grasscloth, textures and some stripes. No repeat allowance; enter a repeat of 0.
The match is printed on the label, often as a symbol.
Accent walls, stairwells and ceilings
A single accent wall is the most common wallpaper job now, and the same method applies with the room perimeter replaced by the wall’s width. A 14 ft wall with 8 ft ceilings and a 21 in repeat needs nine drops, three double rolls, in the wallpaper roll calculator’s run-of-wall mode. Stairwells are harder: each drop is a different length as the stair rises, so measure the longest drop and treat every drop as that length, which over-orders a little but never runs short. Ceilings are papered in drops across the shorter dimension, each the width of the room plus trimming; papering a ceiling is a two-person job and best left to a professional.
Buying: batches and spares
Wallpaper is printed in batches, and colours vary slightly between them. Buy every roll from the same batch (the batch or dye-lot number on the label), and buy a spare if the paper is expensive or the room will need repairs, such as a child’s room or a hallway. A roll from a later batch may not match. Unopened rolls are often returnable; check before buying.
Peel-and-stick
Peel-and-stick wallpaper is counted the same way, in strips, but most products overlap each strip slightly on the next, so the effective width is less than the roll width.
One 12 × 8 ft accent wall, peel-and-stick
20.5 in rolls with a 0.5 in overlap, one window.
- Width covered per strip: 20 in
- Strips needed: 7, two per roll
- Rolls: 4, plus one spare, 5 rolls, $175 at $35 each
From the peel-and-stick calculator.
Borders
A border is counted by length: the run around the room, less any doorways it does not cross, divided by the spool length, typically 15 ft (5 yd) in the US or 5 m in Europe. A 12 × 10 ft room needs four 15 ft spools. The wallpaper border calculator does the arithmetic.
From rolls to cost
Eight double rolls at $45 per single roll is $720 of paper; with paste, primer and tools, a DIY job is about $830, or $2.37 per square foot of wall, from the wallpaper cost calculator. The paint versus wallpaper calculator prices the same room both ways; painting it is $120 in materials. For paste, primer and removal, see wallpaper paste, prep and removal.
Common mistakes
- Dividing wall area by roll area. Count drops; roll ends are waste.
- Forgetting the repeat. It can add a third to the paper.
- Reading a single-roll price as a double-roll price. US prices are usually per single roll.
- Mixing batches. Buy every roll with the same batch number.
- Subtracting whole windows. Paper still runs above and below them.
- No spare roll for an expensive paper. A later batch may not match.