Tile counting looks like simple division but the grout joint changes the answer. A 300 by 600 mm tile with a 3 mm joint occupies 303 by 603 mm on the wall, so its effective area is 0.1827 square metres rather than 0.18. Over a large floor that difference is worth a tile or two.
Cutting waste is the bigger factor. A rectangular room with tiles laid square wastes little, but diagonal layouts, alcoves and doorways push waste well past ten per cent. Trade allowance is 10% for a straightforward room, 15% for diagonal or herringbone work, and 20% where there are many internal corners.
Tiles are sold in boxes, so the practical answer is always a whole number of boxes. This calculator rounds up and shows the spare tiles you will be left with, which is useful: keeping four or five matching tiles for a future repair is far cheaper than trying to source a discontinued batch.
The formula
area- Surface to be tiled, in m²
width, length- Nominal tile dimensions in mm
joint- Grout joint width in mm, added to both tile dimensions
waste- Cutting allowance as a percentage
How it works, step by step
- Enter the area you are tiling, or the length and width of the floor.
- Enter the tile size in millimetres and your grout joint width.
- Set the waste allowance — 10% square, 15% diagonal.
- The gauge shows bare tiles; boxes to order appears below.
Worked examples
12 m² floor in 300 × 600 tiles
With a 3 mm joint each tile covers 0.1827 m², so 12 m² needs 65.7 tiles — 66 whole tiles. A 10% allowance takes that to 73, which is 10 boxes of eight.
Ignoring the joint costs you tiles
Dividing 12 by the nominal 0.18 m² gives 66.7 tiles, which overstates the count by about 1.0. The error runs the wrong way on wide-joint rustic tiles, so always add the joint before dividing.
How to read your score
Frequently asked questions
Does the grout joint really matter?
On small format tiles, yes. A 100 mm mosaic with a 3 mm joint is 103 mm effective — a 6% difference in linear terms. On a 600 mm tile the same joint is only 0.5%, so it barely moves the count.
How much waste should I allow?
Ten per cent for a rectangular room laid square, 15% for a diagonal layout, and 20% where there are many alcoves, pipe boxings or a herringbone pattern. Large format tiles also waste more because each cut discards a bigger piece.
Why do boxes matter more than tiles?
You cannot buy half a box, and part-boxes are rarely returnable once opened. Rounding to whole boxes is the number you actually hand over at the counter, and it usually gives you the spare tiles you want for repairs anyway.
Should I buy extra for a future repair?
Keep at least four full tiles. Production batches shift in shade and tiles get discontinued, so matching a single cracked tile three years later is often impossible.
Tile count reference
| Tile size | Per m² | 5 m² | 12 m² | 20 m² | 40 m² |
|---|---|---|---|---|---|
| 100 × 100 mm | 94.3 | 519 | 1245 | 2074 | 4148 |
| 150 × 150 mm | 42.7 | 235 | 564 | 940 | 1880 |
| 200 × 300 mm | 16.3 | 90 | 215 | 358 | 716 |
| 300 × 300 mm | 10.9 | 60 | 144 | 240 | 480 |
| 300 × 600 mm | 5.5 | 31 | 73 | 121 | 241 |
| 600 × 600 mm | 2.8 | 16 | 37 | 61 | 122 |
| 600 × 1200 mm | 1.4 | 8 | 19 | 31 | 61 |
Per m² figures include the 3 mm joint; area columns include a 10% allowance and round up.