Wallpaper is cut into full-height strips, so roll width, length and pattern repeat drive the quantity.
Why square metres alone are misleading
A roll may contain enough area in theory yet waste a great deal if full-height strips must be cut. Calculate the number of strips, the usable length of each strip and how many fit into a roll.
Worked example
Take a wall 3.80 m wide and 2.50 m high. A roll is 0.53 m wide and 10 m long. Rounding 3.80 ÷ 0.53 upward gives 8 strips. Allowing 10 cm for trimming makes each strip 2.60 m long; a 10 m roll yields 3 full strips. Eight strips therefore require 3 rolls.
Pattern repeats can add waste
If the vertical pattern repeats every 0.64 m, a 2.60 m cut might have to be rounded to 3.20 m to align the design. A 10 m roll still yields three such strips in this example, but other ceiling heights and repeats could reduce that to two.
Should windows be subtracted?
Windows reduce total covered area but do not necessarily reduce the number of full-width strips. Check where strips cross openings and whether offcuts can realistically be reused.
Before purchasing
Check the roll width, length, matching symbols, dye lot and cutting allowance. Measure ceiling height at several points. ParedExacta lets you enter these variables and compare the estimated number of rolls.
Try it in ParedExacta
Enter your own measurements and compare the result. Actual quantities depend on your materials.