Before choosing a wallpaper, keep in mind that wallpaper with little or no pattern to match at the seams and the ceiling will be the easiest to apply, thus resulting in the least amount of wasted wallpaper.
If you do choose a patterned wallpaper, a small repeating pattern will result in less waste than a large repeating pattern.
A pattern that is aligned horizontally (matching on each column of paper) will waste less than one that drops or alternates its pattern (matching on every other column).
To determine the amount of wall space you're covering:
Measure the length of each wall, add those figures together and multiply by the height of the walls to get the area (square footage) of the room.
Calculate the square footage of each door, window or other opening in the room. Add these figures together and subtract the total from the area of the room.
Take that figure and multiply by 1.15, to account for a waste rate of about 15 percent in your project.
Wallpaper is sold in single, double and triple rolls. Coverage can vary, so be sure to refer to the roll's label for the proper square footage. (Average coverage for a double roll, for example, is 56 square feet.) After choosing a paper, divide the coverage figure (from the label) into the total square footage of the room you're papering. Round the answer up to the nearest whole number. This is the number of rolls you need to buy.