The main determinant in ordering the proper quantity of wallpaper for your room is simply the square footage of the room. The easiest way to determine this is to measure each wall separately and add the combined square footage of all the walls.
For example, if an 8’ high wall is 12’ long and has a standard door, the square footage of that wall is: 8’x12’(height x width) = 96 sq.ft. minus the door frame 7’x3’ = 21 sq.ft.; therefore 96sq.ft. – 21 sq.ft. = 75 sq.ft. After adding all four wall measurements you will have the total square footage. Remember to be liberal with your measurements as there is always a potential for waste when trimming the wallpaper. Adding 10% to your order is a good rule of thumb where there is a potential for waste from vertical trimming.
Be aware that when measuring for feature walls, we can only use full and complete strips of wallpaper. First, measure your wall width in inches. When purchasing, determine the width of the wallpaper roll (ie 20.5” or 27”) and then determine how many strips you will need to cover the wall width. For example: you have a wall that is 138” wide (11.5’); your wallpaper is 20.5” wide; you will need 7 full strips of wallpaper (20.5 x 7 = 143.5”) to cover your wall width.