Room perimeter = 2 × (length + width) of the room.
Combined, in metres – whole strips are saved there. Do not subtract normal windows: the pieces above and below still need paper.
Length after which the pattern repeats; 0 = paper without pattern
Standard size 0.53 × 10.05 m – roll area: 5.33 m²
Each strip is cut a little longer than the room height and trimmed at ceiling and skirting. 10 cm (5 cm top and bottom) is our assumption, not a standard – add the pattern repeat for patterned wallpaper.
Rolls needed
13rolls
13For a 20 m perimeter at 2.5 m room height you need 38 strips. With trim allowance each strip is 2.6 m long, one roll gives 3 – that makes 13 rolls.
Only whole strips of room height plus trim allowance count per roll; strips and rolls are rounded up.
What you can do
Keep 1–2 spare rolls from the same batch
The batch number ensures identical colour tones – important for large-scale patterns or complaints.