The calculation works out how many usable drops (vertical lengths) you can cut from a single roll, then divides the total wall perimeter by the roll width to find how many drops you need in total. Pattern repeat adds waste because each drop must start at the same point in the pattern — larger repeats mean more waste per roll.
A standard UK wallpaper roll is 10m long and 52–53cm wide. Always buy one extra roll in the same batch number — colours can vary slightly between print runs, and you may need it for repairs later.
Pattern repeat is the vertical distance before the design repeats. A straight match means each drop starts at the same point — efficient. An offset match (half-drop) means every other drop starts halfway down the repeat — uses significantly more paper. Large or half-drop patterns on tall walls can waste up to 30% of each roll in off-cuts. Always check the label for repeat type and size before buying.
Wallpaper first, then paint woodwork. This is the decorator's standard order: hang wallpaper, trim to skirting and architrave, then paint woodwork with gloss or satin — overlapping slightly onto the wallpaper edge, which gives a cleaner finish than trying to cut paint perfectly to wallpaper. Exception: if you're painting the ceiling, do that before papering the walls.