Interior finishing calculators

Interior work is measured in surfaces rather than volumes, so these calculators all begin from an area and end at a purchase unit that cannot be split — a tin, a box, a sheet. The gap between those two numbers is where the money goes.

Openings are handled differently by each trade, and copying one habit across to another is a reliable way to order wrong. Paint deducts doors and windows, because you genuinely do not paint them. Drywall does not: board is hung straight over an opening and cut out afterwards, so subtracting it leaves you a sheet short. Tile deducts only what is permanently standing on the floor.

Allowances differ too, and for real reasons rather than convention. Paint is governed by how many coats the surface actually needs and how far a gallon stretches on it, which varies more with porosity than with anything else. Tile waste follows the lay pattern, because offcuts are geometry — a diagonal grid produces triangles nothing else can use. Drywall waste follows the room shape, since every short wall and opening leaves a piece too small to hang. None of these three should be given the same flat percentage.