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The Wallpaper Calculator counts double rolls the way paper is actually hung, strip by strip. It divides the room perimeter in inches by the roll width and rounds up to whole strips, makes each strip the wall height plus 4 in — Brewster's hanging instructions add 2 in at the top and bottom for trimming — rounded up to whole pattern repeats, and divides the roll length by the strip length, rounded down, for the full strips each roll gives. Strips divided by strips per roll, rounded up, is the rolls, less Brewster's deduction of 1 roll for every 4 ordinary doors or windows. The default roll is Brewster's US double roll, or bolt, 20.5 in × 33 ft = 56 sq ft; any other roll size can be entered. A 12 × 12 ft room with 8 ft walls needs 29 strips of 100 in, 3 per roll: 10 double rolls, or 9 with two doors and two windows. Brewster's quick chart, which tracks wall area ÷ 56 sq ft, lists 7 for the same room; the page shows both.
Brewster lists a US double roll (bolt) as 20.5 in × 33 ft = 56 sq ft. Strips = room perimeter in inches ÷ roll width, rounded up. Each strip is the wall height plus 4 in for trimming (Brewster), rounded up to whole pattern repeats; full strips per roll = roll length ÷ strip length, rounded down. Rolls = strips ÷ strips per roll, rounded up, less 1 roll for every 4 ordinary doors or windows (Brewster).
| Room | 8 ft walls: strips | 8 ft: Brewster chart | 9 ft walls: strips | 9 ft: Brewster chart |
|---|---|---|---|---|
| 8 × 10 ft | 8 | 5 | 8 | 6 |
| 10 × 10 ft | 8 | 6 | 8 | 6 |
| 10 × 12 ft | 9 | 6 | 9 | 7 |
| 10 × 14 ft | 10 | 7 | 10 | 7 |
| 12 × 12 ft | 10 | 7 | 10 | 7 |
| 12 × 14 ft | 11 | 7 | 11 | 8 |
| 12 × 16 ft | 11 | 8 | 11 | 9 |
| 12 × 18 ft | 12 | 9 | 12 | 9 |
| 12 × 20 ft | 13 | 9 | 13 | 10 |
| 14 × 14 ft | 11 | 8 | 11 | 9 |
| 14 × 16 ft | 12 | 8 | 12 | 9 |
| 14 × 18 ft | 13 | 9 | 13 | 10 |
| 14 × 20 ft | 14 | 9 | 14 | 10 |
| 14 × 22 ft | 15 | 10 | 15 | 11 |
| 16 × 16 ft | 13 | 9 | 13 | 10 |
| 16 × 18 ft | 14 | 9 | 14 | 10 |
| 16 × 22 ft | 15 | 10 | 15 | 12 |
Count the strips and how many fit on each roll. Divide the room perimeter in inches by the roll width for the strips, add 4 in to the wall height for trimming (Brewster's instructions), and divide the roll length by that strip length, rounded down. A 12 × 12 ft room with 8 ft walls needs 29 strips of 100 in; a 33 ft double roll gives 3, so 10 double rolls before deductions.
Brewster lists a US double roll, which it calls a bolt, as 20.5 in × 33 ft = 56 sq ft. Not all of that ends up on the wall, because each strip is cut longer than the wall and the off-cut at the end of the roll is often too short for a full strip.
Brewster's rule is to deduct 1 bolt for every 4 ordinary-size doors or windows. A 12 × 12 ft room with 8 ft walls and two doors and two windows drops from 10 double rolls to 9.
Every strip has to start at the same point in the pattern, so its length is rounded up to a whole number of repeats. With a 21 in repeat an 8 ft wall's 100 in strip becomes 105 in. Large repeats can drop a roll from 3 strips to 2 and add several rolls, which is why Brewster notes that larger pattern repeats need more rolls.
Brewster's quick chart lists 7 bolts for a 12 × 12 ft room with 8 ft ceilings, which is close to the wall area divided by 56 sq ft. The strip count is the conservative figure when every full-height strip has to come from a single roll; short off-cuts can then go above doors and below windows. Buy every roll from the same dye lot.
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