Wallpaper calculator
The count spreads your wall area across rolls after two allowances you control. Pattern matching is unpredictable and papers differ in how they are measured, so order from one batch and buy a spare roll — a shortfall on the last wall means a visible seam of a different dye lot.
Enter the combined wall length and the height, the usable area a roll gives, and two allowances — trim for the top-and-bottom cuts, and a pattern-repeat margin for lining the drops up. The tool works out the area, applies both, divides by a roll and rounds up.
Exemple détaillé
A run of wall 12 ft long and 9 ft high is 108 square feet. Add 10% trim and 15% pattern repeat — 25% together — and you are hanging about 135 sq ft. At 25 usable square feet a roll that is 135 ÷ 25 = 5.4, so 6 rolls. A plain paper with no repeat would drop that toward 5.
In metric: a wall 3.6 m long and 2.7 m high is 9.72 m². With the same 25% it is 12.15 m², and at 4.5 m² usable from a standard 0.53 × 10.05 m roll that is 12.15 ÷ 4.5 = 2.7, so 3 rolls.
Sur le chantier
A whole room. 12 × 10 with 9 ft walls: perimeter 44 ft × 9 = 396 ft². Take off the door (21 ft²) and the window (15 ft²): 360 ft². With 10% trim and a 15% repeat allowance that is 360 × 1.25 = 450 ft², ÷ 25 usable per roll = 18 rolls. Type 44 as the combined wall length and subtract the openings from the area yourself — the tool papers the wall you give it.
Check it by drops. A hanger counts drops, not square feet. Roll 20½ in wide: 44 ft of wall is 528 in ÷ 20.5 = 25.8 → 26 drops. Each drop is the wall height plus up to one repeat for matching: 9 ft + 2 ft = 11 ft, and a 27 ft roll gives floor(27 ÷ 11) = 2 drops. 26 ÷ 2 = 13 rolls. The area method said 18 — it is deliberately fat, because the "usable area" on the label is already discounted and the allowances stack on top. When the two methods disagree this much, the drop count is the truer one; buy it plus one.
The same check in metric: a 0.53 m roll on 13.4 m of wall is 13.4 ÷ 0.53 = 25.3 → 26 drops. With a 2.7 m wall and a 0.53 m repeat each drop is 3.23 m, and a 10.05 m roll gives floor(10.05 ÷ 3.23) = 3 drops: 26 ÷ 3 = 8.7 → 9 rolls.
Same batch, or not at all. Buy every roll at once and check the batch number on the label. A roll from another batch is a different shade, and it shows on the first seam in daylight. The spare roll is for the repair in two years, from the same batch.
What went wrong?
Quick referencefor paper
- Wall area
- perimeter × height − doors and windows
- Allowances
- trim + repeat add — 10% + 15% = 25%
- Drops
- perimeter in inches ÷ roll width · a 20½ in roll: 44 ft of wall = 26 drops · a 0.53 m roll: 13.4 m of wall = 26 drops
- Drops per roll
- floor(roll length ÷ (wall height + one repeat))
- Common rolls
- US double roll 20½ in × 27 ft ≈ 46 ft² raw · metric 0.53 × 10.05 m ≈ 5.3 m²
- Métrique
- 1 m² = 10,76 ft²
Waste & overagewhat to add, and why
- Paperhangers add 10% for trimming and up to 25% more for a large pattern repeat, because every drop starts on the repeat and the piece between is lost — the trim and repeat fields hold both figures separately so you can see which is costing rolls.
- Buy all the rolls from one batch number; a shade difference between batches shows on the wall where two rolls meet.
- Keep a roll for repairs — patterned papers are discontinued quickly.
Mistakes to catchbefore you order
- Typing the roll's gross area when the label gives usable area — some rolls quote the raw size, and the trim eats into it; the field wants usable.
- Deducting doors and windows to the inch; the drop that hits a window still has to be cut from a full length, so most hangers deduct only the big openings.
- Forgetting the repeat allowance on a paper with a big match — the tool's repeat field starts at 15% for a reason.
La formule
rolls = (area × (1 + trim + repeat)) ÷ usable area per roll, rounded up
The wall area is length times height. Both allowances are added together and applied to it, because trim and pattern loss stack. Dividing by the roll's usable area — not the raw roll — gives the roll count, and rounding up turns it into what you buy.
Questions
- How many rolls of wallpaper do I need?
- Total the wall area, add for trim and pattern matching, then divide by the usable area one roll gives and round up. A 12 by 9 foot wall is 108 square feet; at 25 usable square feet a roll, with a 15% pattern-and-trim allowance, you need about 5 rolls. In metric, a 3.6 by 2.7 m wall is 9.72 m²; at 4.5 m² usable a roll with the same 15%, that is 9.72 × 1.15 ÷ 4.5 = 2.5, so 3 rolls. This tool does that once you enter the wall and the roll coverage.
- Why does a patterned paper need more rolls?
- Every drop has to line up with the one beside it, so you cut off part of each length to bring the pattern back to the top — the bigger the repeat, the more you throw away. That is what the pattern-repeat allowance covers. A plain or random-match paper wastes almost nothing, so you can drop that allowance close to zero.
- Should I count the doors and windows?
- For a small opening, leave the wall whole — the trim allowance absorbs it and you keep a margin for mistakes. For a large window or a doorway, subtract that area from the wall you enter so you are not buying rolls to paper a hole. The tool covers exactly the area you give it.
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