Rolls needed for walls.
Rolls needed = (perimeter × wall height ÷ usable sq ft per roll) rounded up.
Standard American double roll is 56 sq ft usable; single 27. Patterns with large repeats need more.
Roll batch numbers matter here too — like tile, wallpaper can show subtle color variation between production runs, so buying all rolls from the same batch avoids a visible mismatch partway across a wall.
40 ft perimeter × 8 ft height = 320 sq ft ÷ 56 sq ft/roll ≈ 6, plus 1 for pattern match = 7 rolls.
Adjacent strips must line up horizontally — larger repeats waste more material.
Small openings: don't subtract (leaves room for error). Large openings: subtract the area from total.
Yes — an extra roll is generally worth having on hand for future repairs, since matching wallpaper years later from a discontinued print or different dye lot can be difficult or impossible.