Rolls of wallpaper typically measure 33ft (10mtrs) long and 21″ (535mm) wide, therefore if the room you are decorating has a ceiling height of approximately 9ft (2760mm) you will be able to cut three drops of wallpaper from each roll. Remember to allow approximately 6″ (150mm) per cut length for trimming the top and bottom of the strip of wallpaper when it is stuck in place on the wall.
To calculate how many rolls you require measure around the walls of the room and divide this total by either 21″ or 535mm.
This figure will tell you how many drops of paper you need, then if you can get three lengths from each roll divide the number of drops by three to find out how many rolls you require. I always purchase an extra roll just in case any mistakes are made or if the wallpaper becomes damaged in the future. I don’t tend to deduct too much paper for doors and windows from the total as it is always better to have more than you need. Most DIY stores will credit you for any rolls you return and there is nothing worse than the store not having any stock of your chosen paper should you run short.
The walls measure 756″ (18900mm) around in total and the ceiling height is 9ft (2760mm) allowing three drops from each roll.
756″ (18900mm) -:- 21″ (535mm) = 36 drops of wallpaper.
36 -:- 3 = 12 rolls.
Therefore with this calculation I would purchase 13 rolls of wallpaper which includes one spare roll for any mishaps etc.
Planning where to start the wallpapering depends on the room itself, if you have a main feature in the room such as a fireplace then this is your starting point (this applies more to patterned paper, but it is something I have always done regardless of the paper being used).
The first strip of wallpaper you hang should be central to the fireplace and needs to be perfectly vertical (this is where the plumb line is invaluable) as this strip determines how straight the other strips will be.
In an empty room with no feature you can start to paper from any point. I usually start from the edge of an architrave, again making sure the first strip of paper being hung is perfectly vertical.
When you buy your favourite wallpaper check the batch numbers or codes on each roll are the same (don’t let this happen to you) as shades and patterns could vary slightly from different production runs when it was manufactured.