A 4 × 3 m floor is 12 square metres, and at 30 × 30 cm each tile covers 0.09 m², so 134 tiles cover it exactly and 147 is what you order. The extra ten per cent is not padding: it is offcuts that cannot be reused, breakage on the saw and in transit, and insurance against a second box being a different shade.
Ten per cent is the working default. Two things push it higher, and knowing which applies before you order is cheaper than discovering it halfway through.
When is ten per cent not enough?
The allowance follows the layout and the room shape rather than the material.
| Situation | Allowance |
|---|---|
| Straight lay, square room | 10% |
| Straight lay, irregular room | 12–15% |
| Diagonal lay | 15% |
| Herringbone or chevron | 15–20% |
| Large-format tiles | 15%; each broken piece costs more area |
| Small mosaic sheets | 10%; offcuts reuse more readily |
The pattern is the biggest single driver. A diagonal lay means every tile at the perimeter is cut at an angle, and an angled offcut fits almost nothing else, so the waste is genuinely a larger fraction rather than an inflated estimate.
Why does the batch matter?
Because tiles are fired in runs and shade varies between them. A box bought three weeks later can be visibly different in the same light, and there is no fixing it short of lifting the affected area.
Wallpaper has the same problem under a different name. Every roll carries a batch number, and rolls from different batches can differ enough to show as a band down a wall. Buying the whole job in one go, from one batch, is the entire answer for both materials.
This is also the real argument for the spare box. Not for the job: for the repair in three years when a tile cracks and the range has been discontinued.
Where does the flooring allowance actually go?
Two details cost more than the cutting. The first is the last row, which almost never falls at a full plank width. A sliver under about 50 mm is both fragile and conspicuous, and the fix is to move the whole floor sideways so the first and last rows share the shortfall evenly. It costs a row of material and it is the single change that most improves how a finished floor reads.
The second is the expansion gap. Floating floors need 10 to 12 mm all round, and it is a functional requirement rather than a tolerance: a floor fitted tight to the walls buckles when the room warms up. That gap comes off the covered area and goes under the skirting.
Diagonal and herringbone lays want 15 to 20 per cent for the same reason as tiles. A 5 × 4 m room is 20 m², and at 1.97 m² per pack that is eleven packs straight and closer to twelve on the diagonal.
Why can grout not be a single multiplication?
Because the joint is a three-dimensional trench rather than a line. What you are filling is a trench with a length, a width and a depth, and the three move independently of one another — which is why no single figure per square metre can be right for more than one tile.
The joint length is the term people underestimate. It is the tile perimeter shared between neighbours, which works out at length plus width divided by area: 6.7 metres of joint in every square metre of 300 mm tiles, 3.3 metres at 600 mm, and eighty metres at 25 mm mosaic. Tile size alone therefore spans a factor of twelve before the joint has been given a width or a depth.
Depth pulls the other way, which is what makes the rules of thumb unreliable rather than merely imprecise. Mosaic is bedded shallow, so its twelve-fold joint length comes back to roughly six times a 300 mm tile at the same joint width — not the four that most coverage tables print. On a 300 mm tile at a 3 mm joint, 8 mm deep, the figure lands near a quarter of a kilo per square metre; double the tile to 600 mm and it halves, because only the joint length changed.
That is why the grout figure has to come from the actual tile size and joint, and why a bag-per-square-metre rule of thumb is wrong for anything but the tile it was written for.
What about the adhesive underneath?
It follows the notch size and the flatness of what you are tiling onto, both of which sit outside the tile calculation. A 6 mm notch on a flat wall uses far less than a 12 mm notch on a floor that needs bedding out, and an uneven substrate can double the figure.
The practical approach is to level the substrate first and then calculate, rather than to use adhesive as a levelling compound. It is more expensive per millimetre than anything designed for the job and it cures at a different rate in thick sections.
Questions people ask
Do I include the grout joint in the tile count? Yes, and it works in your favour slightly — a 30 cm tile with a 3 mm joint occupies 30.3 cm of run, so the true count is marginally lower than area divided by tile area. It is within the waste allowance either way.
Can I lay flooring over existing tiles? Often, if they are sound, flat and the height increase works at the doors. A floating floor is more forgiving here than a glued one.
Should I buy the spare box? For anything that will be difficult to match later, yes. Store it somewhere dry and label it with the room.
How many 30 × 30 tiles per square metre? Eleven and a bit — 1 ÷ 0.09 is 11.1. Ordering by the square metre is safer than ordering by count, because boxes come in whole numbers.
Work out the area, apply the allowance the layout needs, and buy the whole job at once. The tile calculator handles the count and the waste, and the flooring calculator turns an area into packs, boards or rolls — including a grout panel that works from joint width and tile depth rather than from area alone.