Working out how many tiles to buy
Tile quantities come down to two numbers and one judgement. The numbers are the area you are covering and the area one tile covers. The judgement is how much you will cut away, and that depends entirely on the pattern — which is the part almost every calculator flattens into a single figure.
- 144 — square inches in a square foot
- waste — 10% straight, 15% diagonal or offset, 20% herringbone
- tiles per box — printed on the box, and it varies by size and range
Worked example: 100 square feet of 12 inch tile
Ten tiles separate the easiest layout from the hardest on a small floor. On a large one that gap becomes a box or two, which is exactly the margin people run out by when they use a flat ten percent for everything.
Why the allowance follows the pattern
Waste in tiling is offcuts, and offcuts are geometry. A straight lay meets the walls square, so most cuts leave a usable piece for the opposite side. Turn the grid forty-five degrees and every perimeter tile becomes a triangle with nothing reusable behind it. Herringbone cuts at the border constantly and produces awkward slivers that cannot be turned into anything. The pattern is not a style choice with a cost attached — it is the cost.
Common mistakes
- Buying exactly what the maths says. Order the full box, not the tile count. Part boxes are not sold and dye lots change.
- Keeping no spares. A cracked tile in three years is unfixable if the range has been discontinued. Keep a few back.
- Measuring the room, not the tiled area. Baths, shower trays and kitchen units come off the floor figure.
- Applying the straight-lay allowance to a diagonal. The commonest way to end up short by half a box.
When these numbers do not apply
- Rooms far from square. More walls means more cuts. Bay windows and alcoves push waste above the pattern figure.
- Large format tiles. Big tiles waste more when cut, because each offcut represents more square footage.
- Patterned and directional tiles. Anything that has to line up across joints wastes more than a plain tile.
- Mosaic sheets. Sold by the sheet with its own coverage, and cuts fall between the tesserae rather than through them.