+1

TBH I'd just work out the basic size of the walls, as if they were flat, and then add on a rough extra for all of the nooks and crannies judging it by using my years of experience....

tu puta madre

Ahh such a nice turn of phrase, my daughter think's that maybe and just maybe gilipolla¿¿¿¿

Ah but no.

You need to calculate the exact number of coats required in order to cover the existing colour.

Then you need to account for the fact that you will use more paint doing the edges by brush than with a roller in the middle.

It really is a minefield, and a detailed analysis of the problem needs to be carried out in order to establish how much paint and man power will be needed.

I believe thr process is:

look at wall, buy 4 tins of paint, wait 3 months before you start the job, paint walls, think you're running low on paint, buy 4 more tins. FInish job, realise you have 6 tins left & can't return them. Place in garage.

I think you missed a vital step.. get halfway through before wife, mentions she preferes avacodo white with the hint of puce to eggshell green with the undertones of black. redo before getting halfway before wife changes her mind back to the original colour.

Sounds all far too technical to me. I just buy stuff and get on with it then buy more when I run out. Although the last 2 times I did wallpapering I ran out just on the last drop. I resolved this by going back to the store and getting a very long sample

And don't forget it's hard to calculate wall paper as it really does depend on the pattern and how much you have to cut it to match.