How much extra tile to buy, and why small rooms need more

Work out the area, divide by the area one tile covers including its share of the grout joint, then add an allowance for cuts and breakages before rounding up to whole boxes: about 10% for tiles laid straight, 15% for a diagonal, herringbone or a room full of corners. But the allowance is really paying for the cut tiles around the edges, and the edges grow with the room’s perimeter while the tiles grow with its area, so a small room needs a bigger share than a large one. For a small floor, count the tiles row by row instead. Then keep a few spares from the same batch. TILE does the area arithmetic, joint included, with boxes and grout.

The area method, with the joint in

A 600 × 300 mm tile laid with a 3 mm joint takes up 603 × 303 mm of floor, which is 0.1827 m², not 0.18. On large tiles the difference is small; on mosaics and small wall tiles it is several per cent, and leaving it out makes you buy slightly too many.

Take a bathroom floor 2.4 m by 1.8 m, 4.32 m², in those tiles:

The allowance goes on before rounding, not after: 10% of 23.6 is added to 23.6, and the result rounded up once. Rounding up to boxes is where most of the real surplus comes from, and it is worth knowing before deciding whether you need 15%.

Why small rooms need more

Almost every row ends in a cut tile, and the area method quietly assumes that nothing cut off is wasted. The number of edge cuts depends on the length of the walls; the number of whole tiles depends on the floor’s area. Double every dimension of a room and its area goes up four times but its perimeter only twice, so in a small room the cuts are a much larger share of the job, and so is whatever they waste.

How much a cut wastes follows one rule: a piece cut from a tile is reusable only if what is left is at least as big as the next piece you need. If the edge of each row needs a piece less than half a tile long, one tile gives two of them. If it needs more than half, the offcut is too short to use and every row costs a whole tile for its end.

Take a floor 2.2 m wide in the same 600 mm tiles. 2200 ÷ 603 = 3.65, so three whole tiles and a piece of about 390 mm at one wall. That piece is more than half a tile, the 210 mm left over is no use, and every row takes four tiles where the area method counted 3.65: about ten per cent gone on that one edge, before the other wall, breakages or anything else. Set the same floor out from the centre instead, so each side gets a piece of about 195 mm, and one tile yields both: three and a half tiles a row. Where the first tile goes changes what you buy.

On a small room, then, draw the rows, see how big the edge pieces are, count the tiles that way, add one or two for breakages, and only then round up to boxes. On a large, simple floor, 10% is usually generous.

What pushes the figure up

• Diagonal and herringbone. Every tile at the edge is cut at an angle and the offcuts rarely fit elsewhere. Allow 15% or more.
• Large format tiles. A 1200 × 600 tile that cracks during cutting is a big loss, and the offcuts are often too awkward to reuse.
• Obstacles. A toilet, a basin pedestal, pipes, a door frame and a boxed-in soil stack each cost cut tiles.
• Tiles with a pattern or a directional grain that has to run one way, which rules out using an offcut turned round.
• Natural stone, where some pieces are rejected for colour or veining before any are cut.

Keep spares from the same batch

Tiles are fired in batches, and the colour and exact size vary slightly between batches. Boxes are marked with a shade (or batch) code and often a calibre code for the size; check that every box you buy carries the same ones. A tile bought two years later to replace a cracked one may not match, so keep two or three spare tiles, or a part box, somewhere dry once the job is done.

Grout

The grout makers’ formula, printed on most bags, is: kilograms per square metre = (L + W) ÷ (L × W) × joint width × joint depth × density, with the tile’s length and width and the joint’s width and depth in millimetres, and a density of about 1.6 kg a litre for a cement grout. For the bathroom above, with 9 mm deep joints, that is (600 + 300) ÷ (600 × 300) × 3 × 9 × 1.6 = 0.216 kg a square metre, under 1 kg for the floor. An epoxy grout’s bag gives its own figure.

In the calculator

TILE takes the room’s length and width (or an area worked out in AREA for a room that is not a rectangle), the tile size and joint, the allowance and the box size, and gives the tiles, the boxes, the cost if you enter a price, and the grout by the formula above. It shows its working, so the figures can be checked or handed to whoever is laying the floor. It uses the area method, so for a small room, compare its answer with a row-by-row count. The measurements stay in the tab. The calculator is free to use; saving its working is part of OBSCURA Pro.