How Many Tiles Do I Need? Walls, Floors and Wastage

The Calculation Is One Division

Everything about ordering tiles comes down to this:

Tiles needed = area to cover ÷ area of one tile, plus wastage

The arithmetic is trivial. What goes wrong is the units, the wastage allowance, and the fact that tiles are sold in boxes rather than individually. Each of those costs real money, and the last one is the reason people end up with three spare boxes or, worse, half a box short on the final row.

Get the Units Right First

Tile sizes are quoted in millimetres and rooms are measured in metres, which is where a factor of a million can creep in. Convert the tile to square metres before dividing:

A 300 × 300 mm tile is 0.3 × 0.3 = 0.09 m². A 12 m² floor therefore needs 12 ÷ 0.09 = 133.3, so 134 tiles before any wastage.

Or work from tiles per square metre, which is the figure most merchants print on the box:

100 × 100 mm: 100 per m²
150 × 150 mm: 44.4 per m²
200 × 200 mm: 25 per m²
300 × 300 mm: 11.1 per m²
600 × 300 mm: 5.6 per m²
600 × 600 mm: 2.8 per m²

Walls and Floors Are Measured Differently

For a floor, it is length × width.

For walls, it is perimeter × height, which catches people out because the perimeter is not the room area:

A 3 × 3 m room has a 12 m perimeter. At 2.4 m high that is 28.8 m² of wall — more than three times the 9 m² of floor.

Then subtract the openings, but only the large ones. A standard door at 0.8 × 2.0 m removes 1.6 m² and a window at 1.2 × 1.0 m removes 1.2 m², and both are worth deducting. Small openings often save nothing in practice, because the tiles around them still have to be cut from whole ones. If in doubt, deduct the opening and let the wastage allowance absorb the cuts.

Wastage Is Not Optional

Every cut tile produces an offcut that may or may not be usable elsewhere. The allowance depends on the layout rather than the room:

10% — straight, square lay in a simple rectangular room
15% — diagonal or brick-bond lay, or a room with several alcoves and openings
20% — herringbone, patterned tiles that need aligning, or large-format tiles where one bad cut wastes a lot of surface

On that 12 m² floor with 300 mm tiles: 147 tiles at 10%, or 154 at 15%.

Large-format tiles deserve the higher figure even in a plain room. A spoiled 600 × 600 mm tile throws away 0.36 m²; a spoiled 150 mm tile throws away 0.0225 m², sixteen times less.

Two further reasons to err upward. Tiles are produced in batches with slight shade variation, so a top-up order weeks later may not match. And keeping four or five spare tiles means a cracked one can be replaced in ten years without retiling the room.

The Box Problem

Here is the step most calculations skip. Tiles are sold by the box, each covering a set area, and you cannot buy nine and a sixth of a box.

12 m² plus 10% is 13.2 m². A box of sixteen 300 mm tiles covers 1.44 m², so 13.2 ÷ 1.44 = 9.17 — which means 10 boxes, covering 14.4 m².

The rounding has already given you 20% wastage rather than the 10% you asked for. That is usually fine and occasionally decisive: if the next box size up pushes a tight budget over, a slightly different tile with a better box fit can save more than a discount would.

Always check the box coverage in square metres rather than counting tiles, since manufacturers vary the count per box even within one size.

Adhesive and Grout, Which Everyone Forgets

The tiles are rarely the whole bill. Adhesive use depends on the notch depth of the trowel, which depends on the tile:

Small wall tiles, 3 mm notch: around 2.5 kg per m²
Standard wall tiles, 6 mm notch: around 4 kg per m²
Floor tiles, 10 mm notch: around 6 kg per m²
Large format, 12 mm notch or back-buttered: 8 kg per m² or more

So 12 m² of floor needs roughly 72 kg of adhesive — three 20 kg bags, rounded up.

Grout is far less, but it scales with the joint rather than the area: small tiles with narrow joints still use more grout per square metre than large tiles, because there is far more joint length in the same area. Allow 1 to 2 kg per m² for typical wall tiling and more for small mosaics.

Add trim, spacers, a cutting disc and sealant for wet areas, and the non-tile cost can reach a quarter of the job.

Comparing Tiles Honestly

Tiles are advertised both per tile and per square metre, which makes direct comparison harder than it looks. Reduce everything to cost per square metre:

A 300 mm tile at £1.20 each is £13.33 per m². A 600 × 300 mm tile at £4.50 each is £25.00 per m². A 600 mm tile at £9.00 each is also £25.00 per m² — identical cost, despite one tile costing double the other.

Then add the wastage you will actually incur, because a cheaper large-format tile at 20% wastage can cost more delivered than a dearer small one at 10%.

A Sequence That Works

Measure and sketch the area, splitting awkward rooms into rectangles and adding them.
Deduct the large openings on walls; floors rarely need deductions beyond fitted units.
Divide by the tile area in m², or multiply by the tiles per m² figure.
Add wastage: 10%, 15% or 20% depending on the layout and tile size.
Convert to whole boxes using the box coverage, rounding up.
Add adhesive, grout and trim from the area, not from the tile count.

Conclusion

The division is easy; the money is in the three things around it. Convert the tile to square metres so the units cannot bite, pick a wastage figure that matches the layout rather than defaulting to 10%, and round to whole boxes before deciding what the job costs.

Put your room dimensions and tile size into a Tiling Calculator for the count, and reduce competing tiles to a cost per square metre before choosing between them.

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