Wallpaper Calculator Guide. Wallpaper calculations require pattern matching consideration. Formula: Rolls equals Wall Area divided by Usable Area per Roll times Waste Factor. Standard roll: 20.5 inches wide by 33 feet long equals 56 sq ft gross, 48-50 sq ft usable after pattern waste. Pattern repeat affects waste: straight match 10-15 percent, drop match 15-25 percent, large patterns over 24 inches 25-30 percent. Calculate walls: Room Perimeter times Wall Height minus Doors minus Windows. Example 12×15 ft room, 8 ft ceiling: 54 ft perimeter equals 648 inches, times 96 inch height equals 62,208 sq inches equals 432 sq ft. Subtract door 21 sq ft and windows 30 sq ft equals 381 sq ft net. Pattern types: Straight match aligns horizontally, easiest, 10 percent waste. Drop or half-drop match staggers, requires 20 percent waste. Random match or texture minimal 5 percent. Large repeat over 24 inches increases waste significantly. Calculate rolls: 381 sq ft divided by 50 usable equals 7.62 rolls base. Straight match: 7.62 times 1.10 equals 8.4, buy 9 rolls. Drop match: 7.62 times 1.20 equals 9.1, buy 10 rolls. Pattern repeat math: if 24 inch repeat and 96 inch wall: 4 exact repeats, no waste. If 21 inch repeat: 96 divided by 21 equals 4.57, need 5 repeats equals 105 inches per strip, wastes 9 inches per strip. Across 20 strips wastes 180 inches equals 15 feet equals 1 roll. Prepasted activates with water, easier DIY. Unpasted requires separate adhesive, more professional. Peel-and-stick temporary, removable, costs more. Vinyl-coated wipeable for kitchens. Grasscloth and fabric delicate, professional install. Costs: budget 15-30 per roll, designer 40-100, grasscloth 60-200, peel-stick 35-80. Professional install 2-7 per sq ft. Example bedroom: 14×14 ft, 9 ft ceilings. 56 ft perimeter, 504 sq ft walls minus 50 openings equals 454 sq ft. Using 21 inch repeat drop match: 454 divided by 50 equals 9.08 rolls base, times 1.20 equals 10.9, buy 11 rolls at 50 equals 550 materials plus 2,270 install equals 2,820 total. Common mistakes: ignoring repeat adds 2-4 rolls, different dye lots mismatched color, forgetting outlets. Pro tips: order extra for repairs, same lot number, start from focal wall, overlap corners, cut power off for outlets, quality adhesive prevents peeling.