Think in drops, not in square metres
Wallpaper is not paint. You cannot spread it into corners or stretch it over an awkward area. It comes in vertical strips of fixed width, and the only question that matters is how many strips you need and how many strips you can cut from a roll.
A standard European roll is about 10.05 m long and 0.53 m wide, so 5.3 m² on paper. That number is a trap. Whether you get 5.3 m² of wall out of it depends entirely on the arithmetic below.
drops needed = room perimeter ÷ roll width, rounded up
For a 4 × 3.5 m room: perimeter 15 m ÷ 0.53 = 28.3, so 29 drops. Do not deduct for a door or a single window — you will still cut a drop for the wall above and beside them, and the off-cut rarely reaches anywhere useful. Deduct only for something genuinely large: a patio door, a full wall of glass, a chimney breast you are not papering.
How long each drop has to be
Wall height plus trimming allowance. Allow 10 cm — 5 cm at top and bottom — for a plain paper. Ceilings are not level and skirting is not straight; a drop cut exactly to height will be short somewhere.
plain paper: drop length = wall height + 0.10 m
For a 2.7 m wall that is 2.8 m per drop. A 10.05 m roll gives 10.05 ÷ 2.8 = 3.58, so three usable drops. Note the loss already: 1.65 m of the roll, over 16%, is thrown away before the pattern is even considered. This is why rolls never go as far as the square metre figure suggests.
Then the pattern repeat
The repeat is the vertical distance after which the design starts again, printed on the label in centimetres. To make two drops line up, each drop must begin at the same point in the pattern — so every drop after the first is cut to a length rounded up to a whole number of repeats.
patterned drop = ceiling((height + trim) ÷ repeat) × repeat
With a 2.8 m required drop and a 64 cm repeat: 2.8 ÷ 0.64 = 4.38, rounded up to 5 repeats = 3.2 m per drop. A 10.05 m roll now gives only three drops — 10.05 ÷ 3.2 = 3.14 — and 0.45 m of waste on top. On a larger repeat the effect is brutal: a 90 cm repeat forces 3.6 m per drop, which is two drops per roll and 2.85 m thrown away, nearly a third of the roll.
| Repeat | Drop length for a 2.7 m wall | Usable drops per roll | Waste per roll |
|---|---|---|---|
| Plain, no repeat | 2.80 m | 3 | 1.65 m |
| 32 cm | 2.88 m | 3 | 1.41 m |
| 53 cm | 3.18 m | 3 | 0.51 m |
| 64 cm | 3.20 m | 3 | 0.45 m |
| 90 cm | 3.60 m | 2 | 2.85 m |
The counter-intuitive line is the 53 cm one: a moderate repeat can waste less than no repeat at all, because it happens to divide the roll more neatly. This is exactly the kind of result nobody reaches in their head, and the reason to run the numbers rather than to assume patterned always costs more.
Straight match versus offset match
- Free match (plain, random)
- Textures, plains and some grasscloths. Nothing to align, so waste is trimming only. Some of these still have a direction — reverse alternate drops if the label says so.
- Straight match
- The design aligns at the same height on both edges. Every drop starts at the same point in the repeat. Simplest to hang and the arithmetic above applies directly.
- Offset or drop match
- Each drop is shifted by half a repeat relative to its neighbour, so the design runs diagonally. Alternate drops start half a repeat apart, which means you need a half-repeat more length on average and, in practice, that you cut alternately from two rolls. Add roughly half a repeat per drop, or take the pragmatic route and add 15% to the roll count.
The label states which one it is, usually as an icon rather than a word. It also states the repeat and often the offset itself. Photograph the label before you leave the shop; you will want it when the first drop is on the wall.
The full example
A 4 × 3.5 m room, 2.7 m high, straight-match paper with a 64 cm repeat, standard 10.05 × 0.53 m rolls.
perimeter 15 m ÷ 0.53 = 28.3 → 29 drops
drop = ceiling(2.80 ÷ 0.64) × 0.64 = 3.20 m
drops per roll = floor(10.05 ÷ 3.20) = 3
rolls = ceiling(29 ÷ 3) = 10
Ten rolls, for a room whose wall area is about 40 m² and whose rolls nominally cover 5.3 m² each — 53 m² of paper for 40 m² of wall. That gap is not a mistake in the calculation; it is what pattern repeat costs, and it is why estimating wallpaper by square metres is the classic way to end up one roll short.
Practical rules
- Buy every roll in one order and check that the batch number matches on all of them. Colour drift between batches is far more visible on wallpaper than on tile.
- Buy one roll more than the calculation says on anything patterned. Running out mid-wall with a discontinued batch is the worst outcome in this whole guide.
- Measure the wall height in three places. Old rooms vary by several centimetres and the drop must suit the tallest point.
- Start hanging at the most visible corner, not at the door. The inevitable pattern break lands where the last drop meets the first, and you want that behind a door or in a dark corner.
- For a feature wall only, calculate that wall's width alone rather than the room perimeter.
- Keep the last off-cut with the label. Repairs need both.
Roll dimensions vary by market: 10.05 × 0.53 m is the European standard, while American rolls are commonly wider and sold in double or triple rolls. Take the figures from your own label before calculating.