Wallpaper comes in bolts. A bolt of wallpaper (55-60 sq ft) will contain about 40 to 50 usable sq. ft, depending on the size of the pattern repeat. Most wallpaper comes either 20.5" x 33' or 27" x 27' per bolt (grasscloth is different, and comes in bolts that are either 36" x 24', containing 72 sq ft, or 36" x 18', containing 54 sq ft).
There are two popular methods of estimating:
Method #1: Calculate the area of the walls to be covered and divide by the usable sq. ft. per bolt (the usable sq. ft. per bolt estimates can be as low as 40 sq. ft for large repeats of 20-24" to as high as 50 sq. ft for small repeats of 1-2" or random matches); deduct ¼ bolt for each door or window that fully breaks a strip of wallpaper - if a strip of wallpaper goes only partly over a door or window, but with the remainder running all the way down, you would deduct nothing.
Example: To paper an 8' high x 12' long wall with a 20" straight match pattern repeat wallpaper, first calculate the square feet of wall to be covered; 8' x 12' = 96 sq. ft. Allowing 40 sq. ft. per bolt, you would need 96 sq. ft. /40 usable sq. ft per bolt = 2.4 bolt, or, rounding up, three bolts of wallpaper.
Method #2: Calculate the number and length of strips needed (allowing for pattern repeats); then figure how many bolts would be required to yield the strips needed.
Example: To paper an 8' high x 12' long wall with a 20" pattern repeat wallpaper, first calculate the number of strips needed to cover the wall. If the wallpaper comes in 20.5" x 33' bolts, it would take eight strips of wallpaper 20.5" wide to cover a 144" (12') wall. Next figure how many bolts would be required to yield eight strips. Each strip would have to be five pattern repeats (100") to cover the 8' (96") high wall. Since each bolt is 33' (396") long, each bolt would yield three 100" long strips. To get eight strips, then, would require three bolts of wallpaper.