About Interior Walls Calculations
Wall work is an area problem with a twist: openings and pattern repeat. You start with the wall area, which is perimeter times height, then subtract doors and windows. For paint or insulation, that's most of the job. For wallpaper, the pattern repeat changes everything.
A wallpaper roll lists a total square footage, but you never get all of it. Matching a pattern means cutting each strip to line up with the last, and the bigger the repeat, the more you trim off the top of every strip. A 20-inch repeat can waste a foot or more per drop. That's why a room that pencils out to four rolls of plain paper might need five or six of a large-repeat print.
Insulation is sold by the bag or batt to cover a square footage at a target R-value. Wall cavities at 16 inches on center take a standard batt width, so the count follows the wall area minus framing. Spray foam goes by board feet, which is square footage times thickness in inches, and a little goes a long way, so read the kit's yield.
Pros run these numbers to buy one dye lot of wallpaper and avoid a mismatched re-order, and to size an insulation order without leaving cavities short. Figure the area, subtract the openings, add for the repeat, and buy once. Keep a roll back for repairs, since paper lots shift in color.