Every number on this site is an estimate, and every estimate rests on assumptions you can see and change.
Where estimates go wrong
- Material varies. Gravel from two quarries differs in density. Bag yield differs between production runs. Paint coverage depends on what is underneath it.
- Sites vary. A sub-base that is not level takes more material than a flat one. Old rooms are rarely square.
- Waste depends on the method. Tile on a diagonal wastes more than tile laid square.
That is why every constant is editable rather than hidden. If you have a delivery ticket or a data sheet with the real figure, type it in; the answer and the working both follow it.
Codes and safety
Where this site checks a building code, it names the edition and the clause it used and lets you override the limit your local authority enforces. A check it cannot answer says so rather than passing. None of it is a substitute for a licensed trade, an engineer, or your local building department, and for anything electrical, structural or load-bearing, their decision is the one that counts.