These measurements are easiest to take at the top of the baseboard. Measure the height and width of each door, window or other area not to be papered. Now use the convenient wallpaper estimating chart in Fig. 2. This chart is based on 30 square feet per roll. While most manufacturers package 36 square feet per roll, you'll lose about 6 square feet in cutting, matching, trimming, etc.
Many manufacturers pack paper in double-roll bolts.
However, you can still calculate your paper needs by using this chart and then cutting the figure in half where double-roll bolts are used. For example, suppose the room is 10' x 12'. Assuming the ceiling height is 8', the chart shows that 12 single rolls of paper are required. You can deduct a single roll of paper for each two doors or two windows of normal size. It is usually best to start hanging the first strip of wallpaper to the right of a door or window.