Wall Area Is the Wrong Starting Point
Wallpaper is sold in rolls of fixed width and length, and it is hung in vertical strips called drops. The number of rolls follows the number of drops the walls need, and how many drops one roll can yield. Floor area says nothing about either figure. A room of twenty square meters with a two point four meter ceiling needs fewer drops than the same floor area with a three meter ceiling, and the difference in roll count can be three or four.
Measure the total run of wall to be covered, which is the perimeter minus the openings that will not be papered. Measure ceiling height at several points, because floors and ceilings are rarely level, and use the tallest figure. Add a small allowance at the top and bottom for trimming.
Drops Per Roll
A standard roll is fifty-three centimeters wide and ten meters long, though wider and shorter rolls exist. Each drop must be cut to the wall height plus the repeat allowance, so the usable length per drop is the ceiling height rounded up to a multiple of the pattern repeat, plus a trimming margin. Dividing the roll length by that drop length, and rounding down, gives the number of drops one roll yields.
The rounding down matters. A roll of ten meters with a drop length of two point eight eight meters yields three drops with a remainder, and that remainder is usually too short for a fourth drop on the same wall. Treating it as a fourth drop is the most common cause of coming up short at the end of a room.
Pattern Repeat and Half Drop Matching
A straight match repeats at the same height on every strip, so each drop is cut to the same length. A half drop match offsets alternate strips by half a repeat, which means every second drop needs an extra half repeat of length. A repeat of thirty-two centimeters in a half drop pattern effectively adds sixteen centimeters to alternating drops, and the average loss across a wall rises accordingly.
Large repeats waste more. A sixty-four centimeter repeat on a two point seven meter wall rounds the drop up to three point two meters, so a ten meter roll yields three drops instead of three point seven. The same wall with a free match needs no allowance at all and can gain a whole drop per roll.
Deducting Openings
Doors and windows reduce the run of wall but rarely reduce the number of drops. A door of ninety centimeters removes most of one drop width, but the strip beside it still needs cutting and matching. Deducting a full drop for a door and another for each window is closer to reality than subtracting the area. Conservatively, count the drops the openings displace and subtract half a drop for each, keeping the remainder as waste cover.
A Worked Example
A room four by five meters has a perimeter of eighteen meters. Deducting a ninety centimeter door leaves seventeen point one meters of wall to cover. At a roll width of fifty-three centimeters, the wall needs thirty-three drops. The ceiling is two point seven meters and the repeat is thirty-two centimeters, so the drop length rounds up to two point eight eight meters. A ten meter roll yields three drops, and thirty-three drops divided by three gives eleven rolls.
Adding a ten percent allowance for trimming errors and future repairs brings the order to twelve rolls. Buying one extra roll at the start is cheaper than a second order later, because a different batch can differ slightly in shade even with the same pattern number.
Paste, Lining, and Leftovers
Paste-the-wall papers need no adhesive on the back but are less forgiving to reposition, and a lining paper underneath adds a separate calculation based on wall area rather than drops. Keep the batch number from every roll and store the leftover lengths flat rather than rolled, since a rolled edge takes a set that shows on the wall. Label the leftovers with the room name so future patch work matches.