Ordering too little wallpaper causes a delay. Ordering too much wastes money you cannot always get back. This strip method takes about ten minutes with a tape measure and gives you a roll count you can trust.
Measure Total Wall Length and Height
Add up the length of each wall to get the room perimeter, then measure height from the top of the skirting board to the ceiling. A 12ft x 14ft room has a perimeter of 52ft. With an 8ft ceiling, gross wall area is 52 x 8 = 416 sq ft. That is your starting point, not your order quantity. Measure an alcove deeper than about 12 inches as its own section.
Subtract Doors and Windows
Use 21 sq ft for a standard interior door (roughly 3ft x 7ft) and 15 sq ft for a typical window (roughly 3ft x 5ft). For patio sliders or a large picture window, measure the actual width times height. In our example, one door and two windows remove 21 + 15 + 15 = 51 sq ft, leaving 416 – 51 = 365 sq ft to paper.
Count Strips, Not Just Square Feet
Wallpaper hangs in vertical strips, so divide the perimeter in inches by the usable roll width. Most US rolls are 20.5 to 21 inches wide. Here, 52ft is 624 inches. 624 / 20.5 = 30.4, so you need 31 strips. You can check your own figures in seconds using our wallpaper calculator and compare the strip count with the roll count before ordering.
Allow for Pattern Repeat
Find the repeat on the label — commonly 12, 18, 20.5, or 24 inches. Your cut length is the wall height rounded up to the next full repeat, plus 4 to 6 inches for trimming. Straight-match patterns waste least. Drop-match patterns, where the design steps down by half a repeat each strip, waste most, especially with a large repeat on a low ceiling.
Work Out Strips Per Roll
Divide the roll length by your cut length, counting only whole strips. A double roll is 396 inches long. At a 102 inch cut length, that gives 3 strips per roll. Divide total strips by strips per roll and round up: 31 / 3 = 10.3, so 11 double rolls before waste.
Add Waste and Match the Dye Lot
Add 10% waste for a straight match in a simple rectangular room, and 15% for a drop match, a staircase, or a room with many doors and sloped ceilings. Buy every roll from the same dye lot or batch number on the label. Keep one unopened roll, if you can, for future repairs.
Worked Example
Room: 15ft x 11ft, 9ft ceiling, one door, one 6ft x 4ft window. Perimeter is 52ft (624 inches). Gross area: 52 x 9 = 468 sq ft. Subtract 21 sq ft for the door and 24 sq ft for the window: 423 sq ft net. Strips: 624 / 20.5 = 30.4, so 31 strips. With an 18 inch repeat, allow about 114 inches per cut. 396 / 114 = 3 strips per double roll. 31 / 3 = 10.3, round to 11 rolls. Add 10% and order 12 double rolls, all from the same dye lot.
FAQ
Q: Can I order by square feet alone? A: For a rough budget, yes, but strips are more accurate. Two rooms with equal area but different ceiling heights or repeats can differ by two or three rolls.
Q: Should I subtract every small window? A: Subtract anything wider than about 2ft. Leave smaller high windows in the calculation as a built-in allowance for cuts and mistakes.
Q: What if I run short mid-room? A: Reorder the same dye lot straight away. If it has sold out, finish in the least visible corner, where a slight shade shift will show least.