Single rolls, double rolls and what a roll really covers
American wallpaper is priced by the single roll but almost always sold as a double roll: one continuous bolt 20.5 inches wide and 33 feet long, which is 56 sq ft of paper. The equivalent single roll would be 16.5 feet long and 28 sq ft. Wider 27-inch "double rolls" are 27 feet long (about 61 sq ft), and European rolls are usually 21 inches by 33 feet. What you can actually hang is less than the nominal square footage because every strip is trimmed top and bottom and every pattern must be matched, which is why the old rule of thumb counts a single roll as covering only about 25 usable square feet, or roughly 50 per double roll.
This calculator estimates two ways. The area method takes the net wall area (perimeter x height, minus 20 sq ft per door and 15 sq ft per window), adds a waste percentage, and divides by the roll's nominal square footage. The strip method mirrors how a paperhanger works: each strip must be the wall height plus about 4 inches for trimming, rounded up to a whole number of pattern repeats; a roll yields a whole number of such strips; and the number of strips is the wall perimeter divided by the roll width. The tool shows both and recommends the larger, since running one strip short means a second trip and possibly a different dye lot.
Worked example: a 12 x 12 room with 8-foot walls
The perimeter is 48 feet and the walls are 384 sq ft gross. Subtracting one door and two windows leaves 334 sq ft. By area, with 15% waste, that is 384 sq ft of paper, or 384 / 56.4 = 6.8, so 7 double rolls. By strips, each strip is 96 + 4 = 100 inches, a 33-foot (396-inch) roll gives 3 full strips, and 48 feet of wall at 20.5 inches per strip needs 29 strips, so 29 / 3 = 9.7, rounded up to 10 double rolls. The gap between the two answers is real: with 8-foot walls, a third of every roll is unusable end pieces, which the area method cannot see. That is why the strip count is the number to trust.
Add a 24-inch pattern repeat and each strip must be a multiple of 24 inches, so the 100-inch strip becomes 120 inches; a roll still yields 3 strips (396 / 120 = 3.3), so the answer stays at 10 rolls, but a 10-foot wall with the same repeat would need 144-inch strips (124 inches rounded up to six repeats), only 2 per roll, and 15 rolls. Large repeats on tall walls are expensive, which is worth knowing before you fall in love with a pattern.
Buying tips
Order all the rolls at once from the same lot or run number; color can shift slightly between runs and it shows at the seams. Wallpaper is usually returnable in unopened rolls, so rounding up one extra roll is cheap insurance, and a leftover roll is useful for repairs. Peel-and-stick papers are sold in a variety of widths and lengths, so use the custom roll option and enter the size printed on the package. For a single accent wall, switch to the perimeter mode and enter just that wall's width.