🧱 Tile Calculator
How many tiles a surface needs, how many boxes to buy once waste and the grout joint are accounted for, and what it costs. Grout joints matter more than people expect on small formats.
How many tiles, how many boxes, and what the grout costs you
Dividing the floor area by the tile area gives the wrong answer, and it gives it in the direction that leaves you short. Two things are missing: the grout joint, which makes each tile occupy more space than its own size, and the waste from cuts, breakages and the tile you will crack in two years' time.
tiles = area / ((tile width + joint) × (tile height + joint)) × (1 + waste)
The grout joint is not a rounding error
| Tile size | 2 mm joint | 3 mm joint | 5 mm joint |
|---|---|---|---|
| 600 × 600 mm | 0.7% | 1.0% | 1.7% |
| 300 × 300 mm | 1.3% | 2.0% | 3.3% |
| 150 × 150 mm | 2.6% | 3.9% | 6.4% |
| 100 × 100 mm | 4.0% | 5.9% | 9.5% |
| 50 × 50 mm mosaic | 7.8% | 11.4% | 18.1% |
On large-format tiles the joint is negligible. On mosaic it is enormous: a 5 mm joint on 50 mm tiles means nearly a fifth of the surface is grout, and ignoring it leaves you almost 20% short. The smaller the tile, the more the joint matters.
Waste allowances
| Layout and situation | Allowance |
|---|---|
| Straight lay, simple rectangular room | 10% |
| Straight lay with several alcoves or cuts | 12–15% |
| Diagonal lay | 15% |
| Herringbone or chevron | 15–20% |
| Large-format tiles over 600 mm | 15%, breakage is costlier |
| Patterned tiles needing repeat matching | 20% |
Buy the whole job in one batch. Tile is fired in dye lots and a second order weeks later can arrive a visibly different shade, which on a floor is impossible to hide. Keep the leftovers too: the replacement you need in three years will not be available.
Boxes, not tiles
Tile is sold by the box, and boxes do not split. Divide your tile count by the tiles per box and round up, always up. That rounding is itself extra waste: needing 40 tiles with 4 per box is clean, but needing 41 means 11 boxes and 44 tiles. Check the coverage per box, usually printed in square metres, as a cross-check on your own arithmetic.
Adhesive and grout
| Material | Coverage |
|---|---|
| Adhesive, 6 mm notched trowel | 3–4 kg per m² |
| Adhesive, 10 mm notch (large format) | 5–6 kg per m² |
| Grout, 300 × 300 tile, 3 mm joint | 0.4–0.6 kg per m² |
| Grout, 600 × 600 tile, 3 mm joint | 0.2–0.3 kg per m² |
| Grout, mosaic, 3 mm joint | 1.5–2.5 kg per m² |
Grout consumption depends on joint width, tile size and tile thickness together, so these are rough. The pattern is the same as the coverage loss: small tiles need far more grout per square metre.
Setting out
Start from the centre of the room, not from a wall. Walls are rarely straight and almost never square, so starting at one leaves a visibly tapering cut row at the far side. Measuring from the centre puts equal cuts at both edges, which reads as deliberate. Check the room's diagonals before you start: if they differ, you know the cuts will vary and can plan where to hide them.
A worked example
A 4.2 by 3.1 m floor, 13.02 m², with 600 × 600 mm tiles and a 3 mm joint. Each tile effectively occupies 0.603 × 0.603 = 0.3636 m², so 36 tiles cover it exactly. Adding 10% waste gives 40, and at 4 tiles per box that is 10 boxes, 40 tiles, with 4 spare. At €38.50 a box the material comes to €385, plus around 50 kg of adhesive and 4 kg of grout.
If the number here raised another question, it is probably one of these: the square footage calculator or the area calculator.
Frequently Asked Questions (FAQs)
How many tiles do I need?
Divide the surface area by the area each tile occupies including its grout joint, then add waste. For a 13 m² floor with 600 mm tiles and 3 mm joints that is 36 tiles plus 10%, so 40, which means 10 boxes of four.
How much extra should I buy?
Ten per cent for a straight lay in a simple room, 15% for diagonal or large-format, and 20% for herringbone or patterned tile. Rooms with many alcoves and cuts need the higher end, and the box rounding usually adds a little more on top.
Does the grout joint really change the count?
On large tiles barely, on small tiles substantially. A 3 mm joint costs 1% of coverage on 600 mm tiles and 11% on 50 mm mosaic. For anything under about 200 mm, ignoring the joint will leave you short.
Should I buy all the tiles at once?
Yes. Tile is produced in dye lots and a later order can arrive a noticeably different shade. Keep a few spare tiles after the job as well, since the exact product is often discontinued by the time you crack one.
How much adhesive and grout do I need?
Roughly 3 to 4 kg of adhesive per square metre with a 6 mm notch, or 5 to 6 kg for large format with a 10 mm notch. Grout runs from about 0.2 kg per square metre for large tiles to 2 kg or more for mosaic.
Where should I start tiling?
From the centre of the room, so the cut tiles at the edges are equal on opposite sides. Starting from a wall puts all the cutting at the far end, and since walls are rarely square the result tapers visibly.