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.
Eleven and one ninth, so twelve in practice. A 60 × 60 tile is 2.78 per square metre, and a 20 × 20 is 25.
For anything up to a 5 mm joint the difference is under 3% and is swallowed by the waste allowance. For wide joints on rustic tiles, add the joint to each tile dimension.
Roughly 4 kg per square metre for floor tiles at a 6 mm notch, less for wall tiles. Large-format tiles need more because they are back-buttered.
Yes, and keep it. A cracked tile in three years time is a five-minute repair with a matching spare and a very visible one without.