Tile calculator
Ten per cent covers a straight grid in a square room. A diagonal lay wastes far more at the edges — allow twenty. A patterned or rectified tile that has to line up across a doorway wants fifteen. And whatever the number says, buy from one batch: tile colour shifts between batches, and the box you go back for in a month will not match.
Divide the area to be tiled by the area of one tile, then add 10% for cuts and breakages. A 4 × 3 m floor is 12 m²; at 30 × 30 cm each tile covers 0.09 m², so you need 134 tiles before waste and 147 with it.
How to count tiles
The waste allowance is doing more work than it looks. Part of it is genuine offcuts — every tile that meets a wall gets cut and the offcut is usually unusable. Part is breakage, both in transit and on the wet saw. And part is insurance against the batch problem: tiles are fired in runs and shade varies between them, so a box bought later rarely matches. Ordering the extra box now is far cheaper than re-tiling a wall later because the last row is visibly a different grey.
Questions
Ten per cent for a straight lay in a simple rectangular room, fifteen for a room with alcoves or a patterned tile, twenty for a diagonal or herringbone lay.