How the strip method works
Wallpaper is hung in full-height strips, so counting strips is more accurate than dividing square feet. Wall width to cover = the room perimeter minus the width of doors and windows. Strips needed = wall width ÷ roll width, rounded up. Each strip is as long as the wall is high plus one pattern repeat for matching, and strips per roll = roll length ÷ strip length, rounded down. Rolls = strips needed ÷ strips per roll, rounded up.
Worked example: a 12 × 14 ft room with 8 ft walls
The perimeter is 52 ft; minus 6 ft of doors and windows leaves 46 ft (552 in). With a 20.5 in wide roll you need 552 ÷ 20.5 = 26.9, so 27 strips. A 33 ft double roll gives 4 strips of 8 ft, so you need 27 ÷ 4 = 6.75, rounded up to 7 double rolls.
Why pattern repeat matters
A patterned paper has to line up from strip to strip, so each strip is cut longer. In the same room, a 21 in pattern repeat makes each strip 9 ft 9 in long, a 33 ft roll then gives only 3 strips and you need 9 double rolls instead of 7. The repeat is printed on the label.
Single rolls, double rolls and European rolls
In the US, wallpaper is usually priced per single roll but packaged as double rolls: a double roll is typically 20.5 in × 33 ft, about 56 sq ft. A standard European roll is 0.53 × 10.05 m, about 5.3 m². Always enter the width and length printed on the label, and buy all rolls from the same batch number.