Papering a slanted wall, like an attic wall under a sloping ceiling, is harder to estimate than a flat wall, because the height changes from one side to the other. This free calculator handles the slope for you and tells you how many rolls to buy.
A slanted wall is a trapezoid: a rectangle plus a triangle. The tool uses the average of the low height and the high height, times the wall width, which gives the exact area of that shape. Then it subtracts the area of doors and windows, since you do not paper over those.
Rolls are not just about area, though. Wallpaper hangs in vertical strips, and each strip must be at least as long as the tallest part of the wall. The tool divides your roll length by that strip length to find how many strips each roll gives, then divides the wall width by the roll width to find how many strips you need.
Pattern repeat changes the count. If the design repeats every 20 inches, each strip must be cut to a multiple of that repeat so the pattern lines up. That wastes a little paper on every strip, and the tool includes that waste when you enter a repeat.
Measure twice. Measure the width along the floor, the low height at the short side, and the high height at the tall side. If the slope is curved or the wall has dormers and angles, split it into simple shapes, calculate each part, and add them up.
This calculator gives an educational estimate only. Real rolls needed depend on the exact slope, pattern matching, waste, and your supplier roll sizes. Buy one extra roll for mistakes and future repairs, and confirm before buying or building.
Try the example first, then enter your own wall. It is free and needs no sign-up.