THE MATH, MADE VISIBLE
How it works
Wall width to cover = room perimeter, or the width you measure. Openings are subtracted by area and converted back to width: net width = (width × height − openings) ÷ height. Strips needed = round up(net width ÷ roll width). Each strip is wall height + trim + pattern repeat, and strips per roll = round down(roll length ÷ strip length). Rolls = round up(strips including any allowance ÷ strips per roll).
Real project examples
A 12 × 10 ft bedroom
The perimeter is 44 ft; at 8 ft high that is 352 ft². Subtract one 20 ft² door and two 15 ft² windows: 302 ft², or 37.75 ft of full-height wall. With 20.5 in × 33 ft rolls that is 23 strips. With no pattern repeat and 4 in of trim, each strip is 8 ft 4 in, so a roll gives 3 strips: buy 8 rolls.
An accent wall with a large repeat
A 14 ft wall at 9 ft high needs 9 strips of 20.5 in paper. With no repeat, each strip is 9 ft 4 in and a 33 ft roll gives 3, so 3 rolls. With a 24 in repeat, each strip is 11 ft 4 in and a roll gives only 2, so buy 5 rolls.
Before you buy
- Use the roll width, roll length and pattern repeat from the product label or page. A common US double roll is 20.5 in × 33 ft (about 56 ft²), but widths and lengths vary by maker.
- The 4 in trim default follows York Wallcoverings’ hanging instructions, which allow about 2 in at the ceiling and 2 in at the baseboard on each strip.
- Adding one full repeat to every strip is a conservative planning rule. Straight and drop matches waste different amounts in practice, and a random match needs no repeat allowance; enter 0 for random matches.
- Subtracting doors and windows by area assumes offcuts can fill above and below openings. For a cautious order, enter zero openings.
- Check that every roll carries the same run or dye-lot number, and order enough at once. Rolls from a later run can differ in shade.
- Ceilings, borders, sloped walls and stairwells are not included.
Questions worth asking
Why is wallpaper priced as single rolls but sold as doubles?
Many wallpapers are priced per single roll but packaged as double or triple rolls. Enter the width and length of the roll you will actually receive, and a price for that package.
Why does pattern repeat matter so much?
It lengthens every strip, which can reduce how many full strips fit on one roll. In the accent-wall example a 24 in repeat drops each roll from 3 strips to 2 and the order from 3 rolls to 5.
Why is the default extra allowance 0%?
Whole strips, trim and repeat are already built into the count. Add a percentage if you want spare strips for mistakes or future repairs; it is applied to strips before rolls are rounded.
Can I use this for peel-and-stick panels or murals?
Peel-and-stick panels can use the same strip method with their panel width and length. Murals are sold as fixed-size sets; follow the mural maker’s sizing.
Sources & assumptions
NIST · unit conversion factorsYork Wallcoverings · installation guide (strip trim allowance)Wallcoverings Association · roll sizes, repeats and matchesLast reviewed October 1, 2026. Planning defaults are editable examples. Product labels and supplier information override them.