Tiles are rectangles and so are rooms, and the number you buy is decided by how the two rectangles relate rather than by how big the room is. A floor 4.2 m by 3.6 m takes seven 600 mm tiles one way and six the other: forty-two whole tiles, and then the cuts. The cuts are the part that costs money, because a tile cut to fit an edge almost never leaves a piece big enough to use anywhere else. This sets the tiles out on the floor so you can see the grid, then adds the allowance and converts to boxes.
Room, tile size and allowance
The count follows the layout, not the area.
Common sizes: 300, 600, 900 and 1200 mm.
Same as the length for a square tile.
Ten percent is normal for a straight layout; more for diagonal or patterned.
Result
Why area arithmetic gives the wrong answer
Divide the floor area by the tile area and you get forty-two for a 15.12 square metre floor in 600 mm tiles. That happens to be right in this case, and it is right by luck: the room is 4.2 by 3.6, both of which divide by 0.6 almost exactly. Change the room to 4.1 by 3.55 and the same method still says forty-two, while the room actually takes seven across and six down with a thin strip down one side and a thin strip along one end — and both of those strips come out of whole tiles.
The honest way is to work out how many tiles fit each way, round each up separately, and multiply. Rounding up each direction individually is what accounts for the strip at the edge, and it is the reason the answer is sometimes one or two higher than the area method even before any cutting allowance is added.
Where the cutting allowance actually goes
The allowance is not mainly for breakage. It is for the edges, and specifically for the fact that a cut tile leaves a remainder that is only usable if the remainder is big enough to fit somewhere else. A 600 mm tile cut to 200 mm leaves 400 mm, which will fit somewhere. The same tile cut to 500 mm leaves 100 mm, which will not. So the waste depends on where the cuts fall, which depends on where you start laying from.
That is the practical lever: start from the centre of the room, or from the most visible edge, and the cuts fall symmetrically at the skirting where they are least noticeable and most likely to be a usable width. Start from one corner and work across, and you accumulate cut widths that change row by row, which is how a ten-percent job becomes a twenty-percent job.
Tile is made in batches and the shade varies between them. A tile bought two years later to replace a cracked one rarely matches.
Keep six or eight full tiles, boxed and labelled, in the loft or under the stairs. They cost a few dollars now and they are the difference between a repair and a re-floor.
Tile size decides more than the count
Large tiles mean fewer joints, and fewer joints mean less grout, less cleaning and a floor that reads as a single surface. They also mean the substrate has to be flatter, because a 1200 mm tile bridging a hollow will crack where a 300 mm tile would have sat in it. And they mean the adhesive has to be applied to the back of the tile as well as the floor, which changes the material quantities rather more than the tile count does.
Small tiles are more forgiving of an uneven substrate, cheaper to replace, and much easier to lay around obstacles and falls. In a wet room with a floor gully, small tiles are often the only practical choice, because the floor has to be cut into triangles to fall towards the drain and a 600 mm tile cut into a wedge is four hundred dollars worth of offcut.
| Tile | Layout | Allowance | Note |
|---|---|---|---|
| 300 x 300 mm | Straight | 8% | Forgiving; offcuts often usable |
| 600 x 600 mm | Straight | 10% | The default case |
| 600 x 600 mm | Diagonal | 15% | Every edge tile is a triangle |
| 600 x 300 mm | Offset | 10% | Brick-bond; offcuts cascade |
| 1200 x 600 mm | Straight | 12% | Needs a flat substrate and back-buttering |
| Mosaic, on sheet | Straight | 8% | Sheets cut on the net |
A worked example
- The gridA 4.2 x 3.6 m floor takes 7 tiles one way and 6 the other: forty-two whole tiles.
- The allowanceTen percent takes it to 47 tiles.
- BoxesTiles come four to a box, so 12 boxes.
- CostAt $32 a box that is $384 of tile for 15.12 m² of floor.
- What you boughtForty-seven tiles cover 16.92 m² — about 1.8 m² more than the floor, and a good share of that is the offcuts you will reuse.
Forty-seven against forty-two is five tiles of margin, which sounds modest and is about right for a plain rectangular room with no obstacle in it. Add a chimney breast, a soil pipe or a diagonal layout and the same room needs a different allowance.
Laying out before you cut
Dry-lay the floor before you mix anything. Snap chalk lines through the centre of the room in both directions, lay a dry row each way, and look at what the cuts are at every edge. If a cut is going to be under about fifty millimetres at a visible threshold, shift the whole grid by half a tile so the cuts become a tile and a half instead of a sliver. That single decision is worth more than any allowance.
Check the substrate meanwhile: a straightedge laid anywhere on the floor should not rock, and the deviation over two metres should be within about three millimetres for standard tiles and much less for large format. A floor that is out gets a levelling compound first, and levelling compound is a material with its own quantity — the coverage rate figure is the quick way to size it.
Once the tiles are down, the next two quantities are the joint filler and the adhesive. The grout figure works from the tile size and the joint width, and it is surprising how little grout a large tile needs. The adhesive figure works from the trowel notch, which is the thing that actually sets how much you use. If the whole floor is being costed rather than just tiled, the flooring cost figure puts the tile alongside the underlay and the fitting.
Do
- Dry-lay from chalk lines through the centre before mixing anything.
- Shift the grid by half a tile to avoid a sliver at a threshold.
- Buy six to eight spare tiles in the same batch for repairs.
- Check the substrate with a straightedge, especially for large tiles.
- Raise the allowance for diagonal layouts and for rooms full of obstacles.
Avoid
- Dividing floor area by tile area and calling it the order.
- Starting from a corner so the cut widths change row by row.
- Leaving a sliver cut at a doorway where everyone will see it.
- Laying large format on a substrate that has not been levelled.
- Forgetting to order the spare tiles with the job.
Where the numbers stop being true
This treats the room as a plain rectangle with nothing in it. A chimney breast, a soil pipe, a fitted unit or a floor gully all add cuts, and each one tends to remove a tile that would otherwise have been whole and replace it with two pieces that fit nowhere else. Add a couple of tiles per obstacle at minimum.
A diagonal layout is not covered by the allowance alone. Every edge tile on a diagonal floor is a triangle cut from a whole tile, and the other half of that tile is only usable on the opposite edge — so the waste roughly doubles at the perimeter. Fifteen percent is the usual figure and twenty is safer on a small room, where the perimeter is a bigger share of the area.
Nothing here accounts for pattern, for borders, for a feature strip, or for tiles that come with a directional veining that has to be laid in sequence. Any of those turn the job from a count into a setting-out drawing, and the honest answer is to buy twenty percent over and take back what is unused if the supplier accepts returns.
How the figures are produced
Tiles across is the room length divided by the tile length, rounded up, and tiles down is the room width divided by the tile width, rounded up. Their product is the whole tiles that fit. The cutting allowance is applied to that and the result is rounded up to whole tiles, then divided by the tiles per box and rounded up again. The stack compares the area of tile bought with the area of floor, so the offcut share is visible rather than hidden inside a percentage.
Questions people ask
Why not just divide the floor area by the tile area?
Because tiles are rectangular and rooms are rectangular, and the count depends on how the two relate. Rounding up each direction separately is what accounts for the strip at the edge.
How much should I allow for cutting?
Ten percent for a straight layout in a plain room. Fifteen for a diagonal, twelve for large format, and more for a room full of obstacles.
Can I use the offcuts?
Any piece over about a third of a tile, yes, as long as it is square and the edge is clean. Under that, it is usually not worth the time.
Do I need spare tiles?
Yes, six to eight full ones from the same batch. Tile shade varies between production runs and a replacement bought later rarely matches.
Should I start from the centre or a corner?
The centre, on chalk lines. It puts the cuts symmetrically at the skirting and lets you shift the grid to avoid a sliver at a doorway.
Does tile size change the cost much?
The tile count drops sharply as size rises, but the substrate preparation, the adhesive and the handling all go up. Large tiles are rarely cheaper overall.