How this wallpaper calculator works
The calculator multiplies your total wall length by the height, adds a percentage for pattern matching waste, and divides by the usable coverage of one roll. A standard American double roll covers about 57 sq ft; the common European roll (0.53 × 10.05 m) covers about 5.3 m² gross — but pattern repeat eats into both, which is what the waste field accounts for.
Pattern repeat is the hidden cost
A plain or random-match paper wastes little — set the repeat waste to 5%. A straight match with a small repeat wastes about 10–15%, and a large drop-match repeat (over 20 inches / 50 cm) can waste 20–25%, because every strip must be cut to align the motif. The repeat length is printed on the label; the bigger it is relative to your wall height, the more you lose per strip.
Don't subtract doors and windows
Unless an opening is very large, count the wall as solid. The strips around doors and windows still have to run full height for the pattern to match, and the cutouts are rarely reusable elsewhere. This built-in cushion is deliberate — running out of a specific batch is the worst outcome in wallpapering.
Buy the same batch number
Wallpaper is printed in batches (lots), and colors shift subtly between them. Buy all rolls at once and check the batch numbers match. One extra roll is cheap insurance for repairs and mistakes — many retailers accept returns of unopened rolls.