How the wallpaper calculator works
The wall area is twice the room length plus width, multiplied by the wall height: 2 × (length + width) × height. Each door takes 21 ft² off that figure and each window 15 ft², and the result never goes below zero. The coverage of one roll is its width in inches divided by 12 to get feet, multiplied by its length in feet. The waste percentage is taken off the roll coverage, not added to the wall area, because trimming and pattern matching both reduce what can be hung from each roll. Wall area divided by usable coverage, rounded up, gives the rolls to buy.
Example: a 12 × 12 ft room with 8 ft walls is 2 × 24 × 8 = 384 ft². One door and two windows take off 21 + 30 = 51 ft², leaving 333 ft². A roll 20.5 in wide and 33 ft long covers 1.708 × 33 = 56.375 ft². With 15% waste, 56.375 × 0.85 = 47.91875 ft² is usable. Then 333 ÷ 47.91875 = 6.95, so buy 7 rolls.
Roll sizes and pattern repeats
Roll width and length vary between products, so the calculator asks for both instead of assuming a standard size. A wide roll covers more wall per length, and a long roll leaves fewer joins, but neither is better if the pattern does not line up. The waste figure is where pattern matching is allowed for: a small repeat may need 10%, while a large repeat can need 20% or more because each drop is cut to the same point in the pattern. Buy every roll from the same batch or dye lot, since a later roll may not match.
What is not included
The estimate covers the four walls only. It does not paper the ceiling, the door leaves, the window reveals or the inside of cupboards, and it treats the room as a plain rectangle. A room with a chimney breast, sloped ceiling or many corners should be split into rectangles and worked out separately. If the walls need repairing or a skim coat first, the drywall calculator covers sheets for new board, and the paint calculator covers finishing the walls instead.
Frequently asked questions
How many rolls of wallpaper for a 12 × 12 ft room?
With 8 ft walls that is 384 square feet. One door and two windows remove 51 square feet, leaving 333. A 20.5 in × 33 ft roll covers 56.375 square feet, and 15% waste leaves 47.92 usable, so 333 ÷ 47.92 = 6.95, which means 7 rolls.
How much wall does one roll of wallpaper cover?
A roll 20.5 inches wide and 33 feet long covers 20.5 ÷ 12 = 1.708 feet × 33 = 56.375 square feet. Allowing 15% for waste and pattern matching leaves 56.375 × 0.85 = 47.92 square feet of wall.
How many rolls for a 10 × 10 ft room with 8 ft walls?
The four walls are 2 × 20 × 8 = 320 square feet. A 21 in × 32 ft roll covers 1.75 × 32 = 56 square feet, so 320 ÷ 56 = 5.71, which means 6 rolls with no waste allowance.