Two things make wallpaper different from paint and tiles. Both decide how many rolls you need - and getting the number wrong costs you a whole wall.
Here is how it usually goes.
You find a wallpaper you love. You measure the wall, work out the area, and order enough rolls to cover it. You start hanging. Near the last wall, you run out. No problem - you order one more roll.
It arrives. You hang it. And it is slightly the wrong colour.
Not by much. Just enough that you notice it every time you walk into the room.
That is the thing most people don't know about wallpaper. It does not behave like paint, and it does not behave like tiles. Two things make it different, and both of them decide how many rolls you actually need.
The batch
Wallpaper is printed in runs. Each run is called a batch, and it has its own number printed on the label. The colour shifts very slightly from one batch to the next. On a tin of paint that would never matter. On a wall of patterned paper, with the strips side by side, it shows.
So the roll you order three weeks later is almost certainly from a different batch. It will be close. It will not be exact. And once a batch has sold out, there is no way to ask for "the same one as before."
This is why running out is so expensive. You are not simply buying one more roll. You are often re-buying the whole wall so that it matches.
The pattern
Most wallpaper has a pattern that has to line up at every seam. To make it line up, you cut each strip to match the one beside it. That means you lose a little off the top or bottom of every drop.
The bigger the pattern, the more you lose. A small repeat wastes almost nothing. A large repeat can cost you a good piece of every single roll.
This is the part that catches people out. They measure the wall area, divide by the area of a roll, and trust the number. But you cannot tile wallpaper that neatly. The offcuts above a door or below a window usually can't be reused, because the pattern lands in the wrong place. So working from area almost always tells you to buy too little.
The fix is simple
Stop thinking in area. Start thinking in drops.
A drop is one full strip from ceiling to skirting. Work out how many drops your walls need, how many drops you get from one roll once the pattern is taken into account, and you have your real number. Then buy it all at once, in one batch, with a roll to spare.
That spare isn't waste. It is your insurance. If the paper gets scuffed in a year, or a pipe leaks behind it, you have a matching piece sitting in the cupboard.
"I'll just work it out myself"
You can. The width and the height are easy. The pattern repeat is where it slips, and that is the one number that decides whether you run out.
And the shop? A good one will help. But they are working from the figures you give them, they tend to round, and they are not the one standing in front of a half-finished wall when the batch runs dry. The risk is yours, so the number should be yours too.
Get it right once
Wallpaper is one of those jobs where the result is decided before you pick up a brush. Get the number right and you hang it once, it matches, and it looks like it was always meant to be there. Get it wrong and you either redo a wall or live with a seam you can't stop seeing.
That is why I built wallpapercalc.
You give it your wall, your roll size, and the pattern repeat from the label. It counts the drops, adds your spare, and tells you exactly how many rolls to order - all from one batch - so you never have to find out what next month's colour looks like.
Measure once. Order once. Hang it once.
