How the calculation works

Wall area = (room length + room width) × 2 × wall height.
Each door and window is deducted as 20 sq ft.
Single rolls = wall area ÷ usable area per single roll, rounded up. Following Sherwin-Williams' estimating guide, a single roll gives 25 sq ft for repeats up to 6 in, 22 sq ft up to 12 in, 20 sq ft up to 18 in and 18 sq ft up to 23 in — figures that already allow for matching the pattern.
Double rolls = single rolls ÷ 2, rounded up, since most wallpaper is sold in double rolls.

Example

Room length: 12 ft · Room width: 15 ft · Wall height: 8 ft · Doors: 1 · Windows: 2 · Pattern repeat: 15 in

Result: 10 double rolls.

Sherwin-Williams suggests buying one extra roll for repairs.
Make sure every roll has the same run or lot number.
For repeats over 23 inches, use the yield printed on the wallpaper label.
Always confirm coverage and yield with the manufacturer's label — products differ.

Where do I find the pattern repeat?

It is printed on the label or product page, usually in inches. Use 0 for a plain or random-match paper.