Sheets or Tiles Needed (with Waste Allowance)

Divide the area by the area of one sheet, then pad it. The pad is not optional: cuts around openings, breakage, pattern matching and the offcuts that end up too small to use all consume material that never reaches the wall. Ten percent is the standard allowance for a straight run of plywood, drywall or square-laid tile. Go to 15 percent for a diagonal tile layout or a room full of angles, and 20 percent for herringbone or any patterned wood floor.

The arithmetic returns a fraction, and a fraction of a sheet is not something a supplier sells you, so round up and then add one more if the room has a tricky corner. Two cautions on the inputs. Subtract only the large openings from your area, since a small window still forces you to cut a full sheet. And check what a "unit" is at the counter: tile and flooring are sold by the box, and a box that covers 20 ft² is the number you want in $a$, not the size of one tile.