Straight, half-drop and free match
The repeat is the height of one full cycle of the pattern, printed on the label next to the roll size. A straight match lines up across the seam at the same height on every drop. A half-drop lines up halfway: drop two starts half a repeat lower than drop one, drop three matches drop one again.
A free match (stripes, plains, textures) lines up anywhere, so only the trim is wasted. Switch the match above and watch the drops on the wall move.
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.