What the waste percentage is paying for
Three different things, which is why one number feels arbitrary. Off-cuts: every tile cut at a wall leaves a piece too small or too oddly shaped to use elsewhere. Breakage: tiles crack during cutting, during transport and when a full box is dropped on a threshold. Spares: the tiles you should keep after the job, because in five years a cracked tile with no replacement means retiling a floor.
Only the first of the three depends on your layout. The second is roughly constant, the third is a decision. If you separate them mentally you will stop treating 10% as a magic number and start seeing it as three small budgets.
Allowance by pattern
| Layout | Typical allowance | Why |
|---|---|---|
| Straight set, square room | 10% | Cuts only at the perimeter, off-cuts often reusable on the opposite wall |
| Brick / running bond, 50% offset | 10-12% | Every other row starts with a half tile; the cut half is usually usable |
| Brick, one-third offset | 12-15% | Off-cuts are less often the right size to reuse |
| Diagonal, 45° | 15-20% | Every perimeter tile is a triangle, and triangles rarely pair up |
| Herringbone | 15-20% | Two cuts per perimeter piece, both angled |
| Chevron | 20%+ | Mitred ends waste material even away from the walls |
| Modular / multi-size pattern | 15% | The repeat rarely divides neatly into the room |
| Large format, 60 × 60 and above | add 5% to any of the above | One bad cut wastes a large area |
| Mosaic on mesh | 5-10% | Sheets cut between tiles, so off-cuts stay usable |
These are allowances on top of the measured area, not on top of the boxes. Apply the percentage to square metres first, then convert to boxes and round up — doing it the other way round compounds two roundings and buys you a box you did not need.
What the room does to the number
- Squareness
- An out-of-square room means the cut along one wall tapers. A tapered cut cannot pair with anything, so every one of them is a full tile spent. Old buildings are rarely square; measure both diagonals, and if they differ by more than a couple of centimetres add 5%.
- Number of edges
- Waste follows perimeter, not area. A long narrow corridor has far more cutting per square metre than a square room of the same size. Two rooms of 12 m² can honestly need 10% and 18%.
- Obstacles
- Each doorway, pipe, toilet pedestal, kitchen island and floor vent means cuts on more than one side. Count them; four or more in a small room is worth an extra 5%.
- Tile size against room size
- A 60 cm tile in a 3.2 m room leaves a 20 cm strip along one wall, and every tile in that strip is a large tile mostly thrown away. Before buying, divide the room dimensions by the tile dimension and look at the remainder. Sometimes shifting the starting line by half a tile turns two ugly thin strips into two comfortable ones.
- Rectified tiles and thin joints
- Narrow joints leave no room to absorb error, so more cuts have to be exact and more attempts are spoiled.
A worked example
A bathroom floor, 2.4 × 1.8 m, tiled in 30 × 60 cm porcelain, straight set, one doorway and a WC pedestal. Boxes cover 1.44 m².
area = 2.4 × 1.8 = 4.32 m²
allowance = 10% straight set + 5% obstacles = 15%
4.32 × 1.15 = 4.97 m²
4.97 ÷ 1.44 = 3.45 → 4 boxes
Four boxes is 5.76 m², which is 33% over the measured area — considerably more than the 15% you allowed. That is not an error: it is the rounding, and in a small room the rounding always dominates the percentage. It is also why arguing about 10% versus 12% on a bathroom is usually pointless, while on a 60 m² open plan it decides two boxes.
The practical consequence: in small rooms, do not agonise over the percentage, and do keep the leftovers. In large rooms, the percentage is real money and deserves the thought.
Buying rules that matter more than the arithmetic
- Buy the whole job in one order, from one batch. Ceramic colour and size vary measurably between production runs; the batch number is printed on the box and matching it later is luck.
- Check the calibre and shade code on the boxes when they arrive, before the tiler opens the fourth one.
- Open and inspect every box on delivery. Breakage in transit is common and claims have deadlines.
- Keep at least half a box after the job, stored flat and dry. This is the cheapest insurance in the whole project.
- Ask the tiler for their own figure before ordering. Someone who has seen the room will beat any table, including this one.
The allowances here are planning conventions used by suppliers and trades, not standards. They assume a competent installer; a first attempt at herringbone will exceed every figure in the table.