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A wallpaper calculator turns your room measurements into the number of rolls to buy - with extra built in for pattern matching, the thing that trips most people up. Measure the room and it tells you how many rolls to order so you finish the wall without a frustrating half-roll shortfall.
Wallpaper is sold by the roll, and usable coverage is always less than the roll size once you match the pattern at each seam. We add a waste allowance for that, and you should always buy every roll from the same batch (run) number so the colour matches.
Rolls = wall area divided by the usable coverage per roll, times one plus the waste percentage, rounded up. Pattern repeats waste material at every seam, so the allowance matters more than it does for paint - the bigger the repeat, the more you lose.
A room with a 40 ft perimeter and 9 ft walls is 360 sq ft. With 56 sq ft rolls and 15% pattern waste, you need 8 rolls.