Cuts cost more than breakage
The waste allowance on a tiling job is mostly not breakage — it is cuts. Every tile that meets a wall, a window reveal, a socket or a shower tray becomes two pieces, and usually only one is usable.
That is why the allowance scales with the shape of the area rather than its size. A large plain wall wastes proportionally very little. A small bathroom with a window, a basin and a shower enclosure can waste a quarter of every box, because almost every tile in it needs cutting.
The layout matters just as much. A straight grid wastes least. A brick bond wastes more because alternate rows start with a half tile. A diagonal layout cuts every perimeter tile at 45° and both halves are usually scrap.
Buy whole boxes, and buy one spare
Tiles are sold by the box and the box is the unit that matters. The calculator shows both the tile count and the box count so you can see how close you are to a boundary — if you need 101 tiles and the box holds 10, you are buying 110 and that is fine.
One extra box beyond the calculation is worth it on any job you care about. Tiles are batch-fired and shade varies between batches; a cracked tile two years later cannot be matched from a new box with any confidence. Store the spares somewhere dry and flat.
| Layout | Allowance | Note |
|---|---|---|
| Straight grid, plain wall | 10% | The baseline case |
| Brick bond | 10–12% | Alternate rows start with a half |
| Small bathroom, many fittings | 15% | Almost every tile is cut |
| Diagonal | 15–20% | Both halves of a 45° cut usually scrap |
| Large-format (600 mm+) | 15% | A single bad cut wastes a lot of area |
Adhesive and grout are separate
Neither is included here, because both depend on the tile and the substrate rather than the area alone. As a rough guide, wall tiling with a notched trowel uses around 3–4 kg of adhesive per square metre and floor tiling 5–6 kg. Large-format tiles need back-buttering and use considerably more.
Grout depends on joint width and tile size — narrow joints between large tiles use very little, wide joints between small mosaics use a surprising amount. The bag usually carries a coverage table indexed by tile size and joint width; that table is more reliable than any general figure.
Common questions
How many 300 × 300 tiles per square metre?
Eleven and a bit — a 300 mm tile covers 0.09 m², so 11.1 tiles fill a square metre before any allowance for cuts.
Do I need more tiles for a diagonal layout?
Yes, noticeably. Budget 15–20% rather than 10%, because every perimeter tile is cut at an angle and both pieces are usually unusable.
Should I buy a spare box?
On any job you intend to keep, yes. Tiles are batch-fired and a replacement bought later will not reliably match.