Wallpaper looks like an area problem and is not one. You do not hang square metres; you hang vertical strips cut from a roll, each long enough to line its pattern up with the strip beside it. That single constraint — the pattern repeat — is what turns a simple area sum into the estimate people get wrong.
The calculator works the way a decorator does. It takes the room's length, width and wall height, the roll's width and length, and the pattern repeat, and returns the strips to hang, how many whole strips come off one roll, and the rolls to buy. Rolls are sold in whole units from one printing batch, so the answer needs to be right before you leave the shop.
Metric means metres with the roll width and repeat in centimetres; imperial means feet with the roll width and repeat in inches.
strips on a wall = wall length ÷ roll width, rounded uptotal strips = 2 × ceil(length ÷ roll width) + 2 × ceil(width ÷ roll width) — the room is treated as four complete walls.strip length = wall height + pattern repeat, the repeat term appearing only when a repeat has been entered.strips per roll = roll length ÷ strip length, rounded down, and never less than one.rolls = total strips ÷ strips per roll, rounded up.Step 3 is the heart of it. Adding a whole repeat to every strip is the conservative rule: wherever the pattern happens to fall, there is enough length above and below to slide the strip until it registers with its neighbour. A half-drop repeat theoretically needs less, but the tool does not assume you will land on the cheaper side of every roll.
A room 5.0 m by 4.0 m with 2.5 m walls, a roll 53 cm wide and 10 m long, pattern repeat 64 cm.
| Step | Calculation | Result |
|---|---|---|
| Strips, long walls | 2 × ceil(5.0 ÷ 0.53) = 2 × 10 | 20 |
| Strips, short walls | 2 × ceil(4.0 ÷ 0.53) = 2 × 8 | 16 |
| Total strips | 20 + 16 | 36 |
| Strip length | 2.5 + 0.64 | 3.14 m |
| Strips per roll | floor(10 ÷ 3.14) = floor(3.18) | 3 |
| Rolls needed | ceil(36 ÷ 3) | 12 |
With a plain, repeat-free paper the same room needs 36 strips of 2.5 m — four strips per roll, so ceil(36 ÷ 4) = 9 rolls. The pattern alone turns nine rolls into twelve, a third more paper, which is the entire reason this calculator exists.
Repeats also have thresholds, which is why a modest change in the figure can change nothing or change a great deal. A 4.0 m by 3.0 m room with 2.6 m walls and the same roll is 28 strips, three per roll, ten rolls. Enter a 53 cm repeat and the strip becomes 3.13 m — still three per roll, still ten rolls. Push the repeat to 90 cm and the strip becomes 3.5 m, the roll yields only two, and the answer jumps to fourteen. Nothing about the room changed.
Doors and windows are not deducted. The strip count is built from four full walls. An opening may shorten the final strip on that wall, but it rarely saves a whole strip and almost never saves a roll, so treat the strip count as an upper bound and any saving as contingency.
Corners break the pattern. Hanging normally starts in a corner and works outward, and the strip arriving at the next corner is trimmed to whatever width remains. That piece is cut from a whole strip, so every corner costs a full strip even when only 20 cm of it lands on the wall. Rooms with chimney breasts, boxed pipework or a bay need more strips than the four-wall model predicts.
Match type changes the arithmetic. Free match needs 0 in the repeat field, a straight-across match needs the full repeat, and a half-drop match is entered as the same figure. Entering zero for a paper that clearly has a repeat is the most expensive mistake available here, because it under-estimates by roughly a third.
Batch numbers show at the seams. Wallpaper is printed in batches and shade drifts slightly between them — often invisible on a single strip and obvious where two batches meet. Order every roll in one transaction, check the batch or serial numbers agree, and keep one unopened roll from that batch for repairs.
The vertical distance before the pattern repeats itself, printed on the roll label. A 64 cm repeat means the design starts again every 64 cm, so each strip is cut 64 cm longer than the wall to leave room to slide it into register.
Zero. The repeat term disappears from the strip length and the strips per roll rises accordingly. Entering a repeat for a paper that has none can waste a roll in three.
The calculator does not. Deduct a strip only when an opening clears a full strip width across the full wall height, which is rare in a domestic room.
Sometimes. Three strips of 3.14 m from a 10 m roll leave about 0.58 m per roll, and short runs over a door or beneath a sill are the one place those pieces fit. In a room with several such runs, measure them and reduce the roll count by hand.
Because three strips of 3.2 m will not come out of a 10 m roll four times; the fourth piece would be too short to hang. Rounding down is what makes the roll count safe.
Room dimensions in feet, roll width and pattern repeat in inches, roll length in feet. A 13 ft wall takes 8 strips of 21 in paper (13 ft is 156 in, and 156 ÷ 21 = 7.4, rounded up), so a 13 ft by 10 ft room runs to 16 + 12 = 28 strips before any pattern repeat is added. Comparing that strip count against the wall lengths is the quickest way to confirm the fields hold the units you intended.