How to measure a room for wallpaper
Wallpaper square footage is the total vertical surface you plan to cover, measured in square feet. The fastest way to get it is to measure the room's perimeter — walk the base of every wall and add the widths — then multiply by the ceiling height. For an irregular room, or when you're only papering some walls, measure each wall separately as width × height and add the ones you want. This is the same wall area math used for painting, so if you've already measured for paint you're most of the way there.
Once you have the gross wall area, subtract the openings you won't cover. A standard interior door removes roughly 21 square feet and an average window 12–15 square feet. Small windows are often left in for simplicity, since a little extra paper protects you against mistakes, but large picture windows and doorways are worth deducting so you don't over-buy.
Why pattern repeat adds waste
Unlike paint, wallpaper has to line up strip to strip. Every roll lists a pattern repeat — the vertical distance before the design starts again. To match the pattern across a seam, the installer trims off part of each strip, and that trimmed material is waste. A plain or random-match paper wastes very little, but a large drop-match repeat can waste a foot or more per strip, which is why the standard advice is to add 10–15% to your net wall area before converting to rolls.
The taller your walls and the larger the repeat, the closer you should sit to the 15% end of that range. If you're papering a tall stairwell or a room with a big bold motif, some installers add 20%. It's always cheaper to buy one extra roll now than to hunt for a matching dye lot later.
Turning square footage into rolls
Wallpaper is sold by the roll, and US rolls come in two forms: a single roll covers about 28–30 usable square feet, while a double roll — one physical bolt holding twice the length — covers about 56–60. European rolls are narrower and longer, typically covering around 55–56 square feet each. Divide your padded wall area by the usable coverage on the label, not the total printed area, because the first and last strips of every roll usually can't be used in full.
After you know the roll count, you can price the job with the cost calculator, or check the room's floor plan first with the room calculator. Always round the roll count up and set aside one leftover roll from the same batch for future patches.