The formula
drops = perimeter ÷ roll width, and each drop is the height rounded up to the repeat
Where it comes from
Wallpaper is not worked out by area but by drops, and there lies the detail almost nobody allows for: the pattern repeat, the distance at which the design starts again. With a 64 cm repeat, a wall 2.50 m high forces drops of 2.56 — four whole repeats — and the six centimetres over on each one go in the bin. That turns a ten-metre roll from four drops into three, and in an ordinary room that is three more rolls.
How to work it out by hand
- Divide the perimeter by the roll width: those are the drops
- Round the wall height up to the next multiple of the repeat
- Divide the roll length by that figure: drops per roll
- Divide the drops needed by the drops each roll gives
What is worth knowing
Buy every roll at once and check they carry the same batch number: the colour varies between print runs enough to show on the wall. Door and window openings barely save rolls, because the drop has to be cut anyway and what is left above a door rarely fits anywhere else; that is why this calculator takes them off the area but not off the drops. And keep the roll that is left over: it is exactly what will be needed the day a patch of wall gets damaged.