The strip method
Square-foot estimates often come up short because of pattern matching. Instead, work in strips: strips needed = wall width ÷ roll width; each strip's length = wall height + pattern repeat; strips per roll = roll length ÷ strip length (round down). Rolls = strips ÷ strips per roll, rounded up.
Example: 48 ft of wall, 8 ft high, 20.5 in × 33 ft rolls, no repeat: 576 ÷ 20.5 = 28.1 → 29 strips; 396 ÷ 96 = 4 strips per roll; 29 ÷ 4 = 7.25 → 8 rolls.
Single vs. double rolls
U.S. wallpaper is often priced per single roll but sold in double rolls; European rolls are commonly 0.53 × 10 m. Enter the actual length of what you buy.
Tips
Buy all rolls from the same batch/lot number and one extra roll for mistakes. Doors and windows aren't subtracted here; the leftover usually covers above-door pieces.