Rolls needed for your wall — accounts for the pattern repeat, which is where most people under-buy.
Add all walls you're covering
20.5" is most common
33 ft standard, 16.5 ft for many peel & stick
On the label or product page. 0 for solids/textures.
Wallpaper is the one material where guessing is genuinely expensive, because the pattern repeat quietly wastes far more paper than people expect. This counts the rolls properly.
Each strip of paper has to be cut to the wall height plus a trim allowance at top and bottom. If the paper has a pattern repeat, that strip length is then rounded up to the next full repeat, so the pattern lines up between strips — and that rounding is where the waste comes from. The tool works out how many full strips come out of one roll, how many strips the wall needs across its width, and divides. It then adds one spare roll, because running out mid-wall is a far worse outcome than one roll left over.
Example
An 8-foot wall needs a 100-inch strip once trim is allowed. With a 21-inch pattern repeat, each strip must be cut at 105 inches — five full repeats — so every strip throws away 5 inches. A standard roll yields only three strips instead of the four you might expect.
Work out how many strips your wall needs across its width, then how many full-length strips you can cut from one roll after allowing for the pattern repeat, and divide. Add one spare roll.
Every strip has to start at the same point in the pattern so it lines up with its neighbor. That means cutting each strip to a whole number of repeats, and the difference between the wall height and that rounded-up length is thrown away — on every single strip.
One full roll beyond what the math says. It costs a fraction of what re-papering a wall costs when your pattern has been discontinued.