Wallpaper Calculator
How many rolls of wallpaper you need, counted by full-height drops per roll with the pattern repeat, for European and US rolls.
How this is calculated
Wallpaper is hung in drops: full-height strips as wide as the roll. The number of drops is the total wall width divided by the roll width, rounded up. Each drop is cut to the wall height plus one pattern repeat, so the pattern can line up with the next drop.
A roll gives as many whole drops as fit in its length; the leftover piece is too short to use on a full wall. The rolls to buy are the drops needed divided by the drops per roll, rounded up. This is how paperhangers count, and it needs more rolls than dividing the wall area by the roll area, because of the offcuts.
Measure the total width of the walls without subtracting doors and windows; the paper above and below them comes from offcuts and short drops. Buy all rolls from the same batch (dye lot), since colors vary between batches.
Limits
Walls of the same height. For a room with a sloped ceiling or very different wall heights, work out each wall separately. The roll sizes are common sizes; check the label of the paper you choose.