How many tiles you need

Why the waste

During laying some tiles break or need cutting at the edges: add 10% (or more for diagonal patterns or irregular rooms).

Tip

Always buy a few extra boxes from the same batch for future replacements.

Where the waste allowance really comes from

Ten per cent is not superstition: it is what gets lost in the cuts along the walls. In a rectangular room laid straight, the cuts fall on two sides only and the offcut can often be reused on the other side, so even eight per cent is enough. But as soon as the layout changes, everything changes: laid diagonally every row ends with two unusable triangles and you reach fifteen or twenty per cent, and the same goes for a room with recesses, out-of-square corners or pillars.

Format matters too. With large tiles, sixty or ninety centimetres, a bad cut costs far more material than with mosaic, and the percentage has to rise. In a small bathroom full of fittings, where nearly every piece needs cutting, a percentage stops making sense and it is better to draw the layout.

The batch is the thing to check in the shop

Tiles are fired in batches, and two batches of the identical model can carry a different shade: the box states the batch (sometimes «shade» and «calibre»), and mixing two of them in one room shows, especially on pale, uniform colours. Calibre is the second number to check, because it gives the real size of the piece within tolerance: two different calibres will not line the joints up.

Hence the rule every tiler repeats: buy it all at once and buy one box extra. In two years, when a pan falls, that model will be discontinued or from another batch.

Joints are not a detail

A two-millimetre joint on a thirty-centimetre tile adds about seven parts in a thousand to the area covered: very little. But on small formats, with five-millimetre joints, the difference reaches several percentage points, and across a large room that is whole boxes. If the layout uses wide joints, it is better to calculate the area of the module, that is tile plus joint, rather than of the tile alone.

Joints also weigh on the consumables: adhesive, grout and trims are not included in the box count, and in a complete bathroom they can be worth as much as a few square metres of tile.

Two checks before ordering

That the square metres stated on the box match: most manufacturers print pieces per box and square metres, and those are the two numbers to use. And that the room was measured in the right place: measure at floor level and not in mid-air, at two opposite points, because almost no wall is parallel to the one facing it and in an old room the gap can be several centimetres.

When in doubt, round up to whole boxes: leftover tiles live in the cellar, missing tiles stop the job for a week.

Nearby tools

For the area and volume of the room there is Room area and volume; for painting How much paint you need and for decorated walls How much wallpaper you need. For areas in general Areas and perimeters.