Dropmatch

A patterned wallpaper needs more than "width times height" says, because every strip has to line up and the offcut is waste. Dropmatch works out the drops, the rolls, the waste and a spare, and shows how a straight match and a half-drop differ.

The walls

For a whole room, add up every wall width right round, doors and windows included. Height is floor to ceiling (or coving).

The roll

Most UK and EU rolls are 10.05 m long and 53 cm wide. Choose Custom to type an odd size. Always confirm it on the roll's own label.

The pattern

The match type and repeat are on the roll's label. Trim is the spare at top and bottom to cut off (about 100 mm). The spare margin keeps a little back for mistakes and repairs.

Why a pattern costs you rolls

Wallpaper goes up in vertical strips, called drops. The number of drops is the wall width divided by the roll width, rounded up; how many drops you get from a roll is the roll length divided by the length of one drop, rounded down. On a plain paper a drop is just the wall height plus a little to trim top and bottom. On a patterned paper every drop has to begin at the same point in the design so the strips line up, so each drop is cut up to a whole pattern repeat longer than the wall is tall, and that extra is waste. A straight match lines the pattern up at the same height across the join; a half-drop (offset) match steps it down by half a repeat on every other strip, which lines up on the diagonal and wastes more again. These are planning figures: read the length, width, repeat and match off your roll's own label, and buy every roll from the same batch so the colour matches.