Measure twice.
Order once.

Every strip of wallpaper has to line up horizontally with the strip next to it. If a pattern repeats every 21 inches, the installer has to slide the next strip up or down within that 21-inch window until the motif matches.

That slid-off length becomes cut waste at the top or bottom. A wall with a 21" repeat can lose a foot of usable material per strip — which is why our calculator adds the repeat into the strip-length calc, not just sticks a flat 15% on at the end.

In the US, almost everything is sold as a double roll — a single bolt that's roughly 27" wide × 27 ft long, or about 56 square feet of usable paper. A "single roll" is half of that, and was the old standard before wider rolls became common.

European rolls run narrower and shorter (about 20.5" × 33 ft). Commercial Type II vinyl runs 36"–54" wide and is sold by the yard. The calculator handles all five common sizes — pick whatever your specific pattern lists on its product page.

Yes — without exception. Each production run of wallpaper goes through the printer as a single "dye lot" or "run number." Different lots can have subtle but visible color shifts that show up as a vertical seam line on your wall.

Always order one or two extra bolts up front, all from the same lot. Re-orders placed a week later almost always ship from a different lot and can't be matched. The estimate above already factors a 15% buffer — but order another bolt anyway if the room is large.

The default 15% is the right number for most rooms with a standard repeat and a competent installer. It covers trim at the ceiling and baseboard, mistakes around outlets, and one or two unusable strips.

Bump it to 20% if the pattern repeat is over 21", if you're a first-time DIYer, or if the room has lots of angles and built-ins. Drop it to 10% only for plain or random-match papers (grasscloth, solid linens) where strips don't have to align.