Most popular are American Rolls and European Rolls . The difference is that American Rolls are approximately 27 inches wide and Metric Rolls are about 21 inches wide. This makes a big difference in the amount of square footage you'll get out of one roll or the other. As wallpaper passes over the print rollers it repeats itself on a regular basis. Because of this repeating pattern, the person hanging the wallpaper must align the wallpaper to the repeating elements in order to look right and any excess must be trimmed away from the top and bottom edges.
Longer repeats require trimming away more of the paper and result in more waste.
The amount of paper that is actually used and applied to the wall is called usable yield. There are a number of different ways to estimate how much wallpaper you are going to need. On this site I will also give you another chart to help figure it out. To use the chart below, you will need to know how much square footage of wall space you are going to cover. I suggest you do not subtract windows and doors from the total unless they make up a significant amount of area.