Wallpaper transforms a room in a way paint simply cannot, but it punishes you for buying the wrong amount. Unlike paint, you cannot easily run back to the store for one more roll. Dye lots change, patterns get discontinued, and you end up with a seam that doesn't quite match. Getting your roll count right before you order is the single most important step in any wallpaper project.
The pattern repeat is the vertical distance before the design starts over. It is printed on the label and determines how much wallpaper you waste when lining up seams. A 0-inch repeat (also called random match) means you can butt any two edges together, so there is virtually no waste. A straight match means every strip starts at the same point in the pattern, so you waste the distance from the end of one strip to the next full repeat. A drop match (often half-drop) means alternating strips start at different points in the pattern, which can waste up to two-thirds of a repeat per strip in a worst-case scenario.
Strips are aligned so the pattern is at the same height on both sides of every seam. This is the most common type for geometric and simple floral patterns. Waste per strip equals the pattern repeat divided by two on average.
Every other strip drops down by half the repeat height to create a diagonal pattern flow. Common in large-scale prints. This style requires significantly more paper than straight match at the same repeat size. Always add at least 20% extra rolls when working with a large drop-match pattern.
Textures, grasscloth, and many solid wallpapers have no repeat at all. You can cut strips to length with minimal waste, making these the most economical options for odd-shaped rooms or rooms with lots of windows and doors.
Measure the perimeter of the room in feet and multiply by the ceiling height to get total wall square footage. Then subtract the square footage of doors and windows you will not be papering. A standard door is about 20 square feet, a window about 15. Keep a running note of your pattern repeat before plugging the numbers into a calculator, as it directly affects how many rolls the estimate will require. Most standard American single rolls cover about 35 square feet of usable area after accounting for trimming and matching. European single rolls are narrower and shorter, about 28 square feet usable. Check the label carefully.
After calculating your adjusted square footage (wall area minus openings), divide by the usable square footage per roll for your chosen wallpaper. Round up to the nearest whole roll, then add one additional roll as a safety margin. For a room with a large pattern repeat or a drop match, add two extra rolls instead of one. Order all rolls from the same production run. The run number is printed on the label alongside the color code. Rolls from different runs can have subtle color shifts that are invisible in the store but obvious on your wall.
Keep every leftover roll in a cool, dry place. Wallpaper is notoriously difficult to patch after the fact because the existing paper will have faded or shifted slightly. Having an extra roll from the same dye lot is your only insurance policy if a section gets damaged years later.
The condition of your walls determines how good the finished result looks. Fill any holes and sand smooth. Remove switch plate and outlet covers. If the existing walls are bare drywall, apply a wallpaper primer (also called a sizing) to seal the paper facing and give the paste something to grip. On previously painted walls, clean off grease and dust, and lightly sand glossy surfaces. Removing old wallpaper before hanging new is almost always the right move; papering over existing wallpaper can cause the old layer to bubble or peel, taking your new paper with it.
Start from a plumb vertical line in the least visible corner of the room. Laser levels make this easy. Work outward in both directions so any pattern mismatch ends up in the corner you started from. Apply paste evenly and allow the paper to book (fold pasted sides together) for the time specified by the manufacturer; skipping this step causes bubbling. Smooth strips from the center outward with a wallpaper brush or plastic smoother to eliminate air pockets. Trim top and bottom edges with a sharp utility knife and a broad knife as a guide. Change blades frequently. A dull blade tears rather than cuts cleanly.
The most common mistake is not plumbing your first strip. Walls are rarely perfectly square, and a slightly off-plumb starting strip will throw every subsequent strip further out of alignment. The second most common mistake is rushing the booking time, which leads to bubbles that are difficult to remove after the paste sets. Finally, do not overlap seams. Butt them tightly edge-to-edge. Overlapping creates a visible ridge and is almost impossible to fix once the paste dries.
