Measuring walls
Measure each wall’s length along the floor and its height from floor to ceiling. For a rectangular room, the total wall length is twice the length plus twice the width. Measure doors and windows separately so they can be subtracted, and decide up front whether the ceiling is part of the job. The wallpaper calculator counts full-height drops around the room, and the plaster calculator works from the coverage per bag on your product’s data sheet.
Sheets and offcuts
Sheet goods are bought whole, so the calculators round up to full sheets and show how much of what you buy ends up as offcuts. The estimate is based on area: the real count depends on sheet orientation, framing layout and how many offcuts you can reuse. Compare sheet sizes your supplier stocks before you buy.