Retailers disagree about subtracting openings, and the disagreement is real. When deducting is safe, when it quietly under-orders patterned paper, and what the calculator does by default.
Nine rolls, under stated assumptions. A room-size chart from a powder room to a large living room, with the drops behind every number and the four things that change it.
Measure the walls, read the label, pick the match, and count drops instead of square feet. The method installers use, worked on a real room, with the area shortcut shown beside it so you can see where it under-orders.
Why American wallpaper is priced by the single roll and sold by the double, what each one actually holds, and how to avoid ordering half or twice what your walls need.
Why wallpaper comes in two unit systems, what a 53 cm × 10 m roll and a 27 in × 27 ft double roll each hold, the conversions you actually need, and which calculator to use.
What the vertical repeat on a wallpaper label means, how random, straight, and half-drop matches differ, and exactly how each one changes the length of every cut and the number of rolls you order.
Why every wallpaper estimate carries a waste allowance, where the trade's 10% and 20% figures come from, when to raise yours, and how to keep waste from being counted twice against doors and windows.