Most wallpaper calculators divide the wall area by the roll area and get it wrong. The pattern repeat is what decides it: with a 64 cm repeat on a 2.4 m wall you lose most of a drop every time, and that is the difference between four rolls and six.
Notes
Why the repeat changes everything
Each drop has to start at the same point in the pattern, so you cut to the next whole repeat above the wall height. A 2400 wall with a 640 repeat means cutting drops of 2560 — 160 wasted on every single drop.
Straight match and offset match
A straight match lines up across the seam at the same height. An offset or drop match shifts by half a repeat each drop, which means you alternate between two starting points — it wastes less overall, because the offcut of one drop starts the next.
Always buy from one batch
Rolls printed in different batches differ slightly in colour, and the seam between them is visible on a wall. Buy the extra roll now; matching a batch later is usually impossible.
What this does not do
It counts drops on plain walls. Alcoves, chimney breasts, stairwells and papering around windows all change the count, and stairwells in particular waste far more than the arithmetic suggests.