[header] How to estimate the amount of wallpaper needed [title] Calculate the area of the project. [substeps] Measure the length and height of each wall you wish to cover in wallpaper. Subtract doorways and large windows from the total length. 
You can round the length measurement up to the nearest half foot or foot (or meter) to make it easier to calculate. Multiply the length and width of each wall to get the area in square feet.