The method

How this calculator works

Count full drops across the walls

Wallpaper is installed in vertical lengths called drops. Dividing the combined wall width by the roll width gives the number of drops; the calculator rounds up because a partial final width still needs a full drop before trimming.

Measure each wall at the widest point and add them together. Leaving windows and doors in the total is a cautious method because paper around openings still needs matching, and offcuts are not always reusable elsewhere.

Allow the pattern to line up

A straight-match or offset pattern can make each cut length longer than the wall. The calculator adds 10 cm for trimming, then rounds the height up to the next complete repeat. For example, a 240 cm wall with a 53 cm repeat becomes 250 cm, which rounds up to a 265 cm drop.

Check the label for both pattern repeat and match type. A half-drop match may create different alternating cut lengths, so the result should be treated as a baseline and checked with the retailer’s batch guidance.

Buy one batch where possible

Wallpaper colour can vary slightly between production batches. Once the number of rolls is known, try to buy all of them with the same batch code. An unopened spare roll is often more useful than trying to source a matching batch halfway through the job.

A 10% allowance is reasonable for a straightforward rectangular room. Increase it for large repeats, complex corners, sloped ceilings, fragile paper or a first-time installation.

Worked example

Example: 12 m of wall at 2.4 m high

  1. With 52 cm-wide paper, 1,200 ÷ 52 = 23.1, which rounds up to 24 drops.
  2. Add 10 cm for trimming (250 cm), then round up to the next 53 cm repeat: five repeats make a 265 cm drop.
  3. A 10 m roll gives 3 drops (3 × 265 cm = 7.95 m; the last 2.05 m is too short for a fourth).
  4. 24 drops need 8 rolls. A 10% allowance adds 3 spare drops (27 in total), which makes 9 rolls.

Practical answer: Buy 9 rolls: 8 cover the 24 drops exactly and the ninth is your allowance for pattern mistakes and damaged lengths. A difficult half-drop pattern may justify confirming the quantity with the supplier.

Before you buy

Common measuring mistakes

  • Calculating only square metres and ignoring the number of full vertical drops.
  • Forgetting to round each drop up for the printed pattern repeat.
  • Mixing rolls from different batch numbers without checking the colour match.
  • Ordering the exact theoretical amount with no allowance for trimming or a damaged length.