Generally, in order to calculate how much wallpaper you need, use the following steps:
First, calculate the wall area by multiplying the height of the wall by the width.
Second, calculate the area of the wall you will not be papering (i.e. windows and doors) by multiplying the height by the width of each area you will not paper.
Third, subtract the unpapered area from the area of the wall. This will give you the wallpapering area.
Fourth, divide the wallpapering area by the square footage of the wallpaper you would like to purchase.
This will give you the total number of rolls needed to complete your project. However, you will need to consider extra paper for the pattern repeat and extra trimmings.