Wallpaper is hung in vertical strips. The number of strips is wall width in inches divided by roll width, rounded up. Each strip is cut a little longer than the wall height so you can trim. Pattern repeats force you to round that cut length up to the next multiple of the repeat.
A standard U.S. roll is often about 20.5 inches wide and 33 feet long (about 56 square feet). European metric rolls differ—enter the dimensions on your roll.
Subtract large doors and windows so you do not buy paper you will throw away, then keep a waste factor for mistakes and matching.
Drapes on the same window are the curtain fabric calculator. Fresh board before hanging paper is the drywall calculator, and a tiled splash uses the tile calculator.
How this calculation works
strips = ceil(wall width in ÷ roll width in). Cut length includes trim and pattern repeat. Rolls = ceil(total strip length ÷ roll length).