Wallpaper Calculator – Count Drops, Not Square Metres
Most online estimates divide the wall area by the area of a roll and add 15%. That is quick, but wallpaper is not sold by area. It comes in rolls of a fixed width and length, every strip has to run the full height of the wall, and each strip must line up with the pattern of the one beside it. This wallpaper calculator works the way a decorator does: it splits each wall into drops (strips), works out how long each cut is, and then simulates cutting those drops from real rolls, pattern alignment included. The number of rolls that simulation uses is how many rolls of wallpaper you need.
The Drop Method vs the Area Method
A wall 4 m wide needs ceil(400 ÷ 53) = 8 drops of 53 cm paper. With a 2.5 m height and 10 cm trim, each cut is 260 cm, and a 10.05 m roll holds only three of them, leaving about 1.8 m that is too short for a fourth. Eight drops at three per roll is three rolls. The area method never sees that leftover, which is why it tends to under-buy for tall walls and large pattern repeats. The calculator still shows the quick area figure next to the drop count, so you can see the difference for your own room.
Pattern Match Types
Straight match
Half-drop match
A random or free match (plains, textures, most stripes) can be cut anywhere on the roll. A straight match lines up horizontally, so every drop has to start at the same point in the design. In a half-drop match the design runs diagonally and every second drop is shifted down by half a repeat. The pattern repeat is the vertical distance before the design repeats, and it is printed on the roll label. After each cut you may have to skip part of a repeat before the next drop can start. That skipped paper is the pattern-match waste shown hatched in the roll cutting plan.
Rolls also do not start at a known point in the design. With Reserve one pattern repeat turned on, the calculator allows one full repeat at the start of each roll for finding the match, which covers the worst case.
Doors, Windows and Why They Rarely Save a Roll
An opening only saves paper when a whole strip lies inside its width. That strip is replaced by a short piece above the opening and, for a window, another below it, and these short pieces are usually cut from leftover roll ends. A strip that is only partly over the door is still hung full length and trimmed round the frame. Where the opening sits matters, so enter its distance from the left edge of the wall. The wall diagrams show exactly which strips are saved.
Measuring Tips
- Measure each wall at its widest point, and the height at its tallest, from the floor or skirting to the ceiling or cornice.
- Measure openings including their frames, and note how far each one is from the left corner.
- Use Custom walls for chimney breasts, alcoves and walls of different heights.
- For a ceiling, strips run across the shorter side of the room, which keeps drops short and easier to hang.
Paste-the-Wall vs Paste-the-Paper
Many modern non-woven papers are paste-the-wall: you roll adhesive onto the wall and hang the paper dry, which makes cutting drops straight from the roll easy. Traditional papers are paste-the-paper and usually need a soaking time before hanging, so it helps to cut and number several drops in advance, following the cut list. Either way, always follow the instructions on the roll label.