Count drops, not square metres
Wallpaper goes up in full-height drops, so the count starts with how many drops each wall takes: its width divided by the roll width, rounded up. Every wall starts a fresh drop at the corner.
Then each drop is cut from the roll at the point where the pattern lines up with the one before it. On a 64 cm straight match, a 2.55 m drop uses 2.56 m of roll; on a half-drop, every second drop starts half a repeat lower. Repeat cuts every drop in order and draws the rolls, so you can see the offcuts before you buy.
Questions
How do I work out how many rolls of wallpaper I need?
Divide each wall's width by the roll width and round up to get its drops. Add a trim allowance to the wall height and round it up to the pattern repeat to get the cut length. Count how many cuts fit on one roll, round down, then divide the drops by that and round up. A 3.6 × 3 m room with 2.5 m walls takes 26 drops and 9 rolls of 53 cm paper with a 64 cm repeat.
Why does the pattern repeat change the number of rolls?
Each drop has to start where the pattern lines up with the drop next to it, so the paper between the end of one cut and the next matching point is waste. On a 10.05 m roll that can be the difference between four drops and three, which turns 7 rolls into 9.
What is a half-drop match?
The pattern lines up halfway down: every second drop starts half a repeat lower than the one beside it. It usually wastes more than a straight match, and it is why calculators that ignore the match run short.
Should I subtract doors and windows?
No. Paper goes up in full drops and is cut around openings, and the short pieces above doors rarely match from offcuts. Graham & Brown, Thibaut and most decorators say to measure the full wall.
How much extra wallpaper should I buy?
Buy the rolls Repeat counts plus one spare from the same batch for mistakes and repairs later. Unopened rolls can usually be returned; a roll from a different batch can be a shade off.
What size is a roll of wallpaper?
A standard European roll is 53 cm wide and 10.05 m long, wide rolls are 70 cm. In the US, rolls are priced as single rolls and sold as double rolls, about 20.5 in by 33 ft. Check the label: width, length, repeat and match are all printed on it.