The calculation
Total area ÷ area of one tile × (1 + waste) = tiles to buy. Everything hinges on getting both areas into the same units, which is where most errors happen — tiles are sold in inches or centimetres while rooms are measured in feet or metres.
A 12 × 15 ft room is 180 ft². A 12 × 24 inch tile is 2 ft². That is 90 tiles bare, 100 tiles with 10% waste, or 13 boxes of 8.
Waste is not optional
Every tile cut at a wall leaves an offcut that is usually unusable, and some tiles break. The right allowance depends on the layout, not on how careful you are:
- 10% — straight grid layout in a simple rectangular room
- 15% — diagonal layouts, or rooms with alcoves, angles and many edges
- 20% — herringbone, chevron and other patterns with many angled cuts
- Add more for large-format tiles, where a single bad cut wastes a lot of surface area
Buy the whole job at once
Tiles are made in batches, and colour varies measurably between them. Buying a top-up box later often produces a visible mismatch across a floor in daylight. Buy the full quantity, from the same batch, in one order — and keep the spares, because a cracked tile in three years is otherwise unrepairable.
Grout lines and layout
Grout joints slightly reduce the number of tiles needed, but only by 1 to 3% for typical joint widths — well inside the waste allowance, which is why this calculator ignores them rather than giving false precision. What matters far more is where you start the layout: centring the grid means symmetrical cuts at both walls, while starting flush against one wall can leave a sliver of tile at the other.
Doing the subfloor too? The concrete calculator handles slabs and screeds with the same measure-then-add-waste approach.