Work in square metres, not tiles
The instinct is to count tiles. Do not: tile sizes vary, boxes hold different quantities, and cut tiles do not divide neatly. Everything in tiling is sold and specified by the square metre, so measure in square metres and convert to boxes at the very end.
A bathroom is several separate surfaces rather than one. Measure each and add them together:
- Each wall you are tiling: width x tiled height
- The floor: length x width
- The bath panel, if it is being tiled
- Any window reveals and the top of a boxed-in pipe run
Tiled height matters as much as the room size. A full-height wall behind a shower, a half-height splashback elsewhere, and a single row above the basin are three different numbers, and using the ceiling height for all of them is the most common way people over-order by half.
A worked example
Take a bathroom 2.0 m wide by 2.2 m long, 2.4 m to the ceiling. The shower wall and the bath wall are tiled floor to ceiling, the other two walls get a 1.2 m half-height, and the floor is tiled throughout.
| Surface | Sum | Area |
|---|---|---|
| Shower wall, full height | 2.0 x 2.4 | 4.80 m2 |
| Bath wall, full height | 2.2 x 2.4 | 5.28 m2 |
| Two walls, half height | (2.0 + 2.2) x 1.2 | 5.04 m2 |
| Floor | 2.0 x 2.2 | 4.40 m2 |
| Less door and window | roughly | -2.60 m2 |
| Subtotal | 16.92 m2 | |
| Plus 10 percent waste | 16.92 x 1.1 | 18.61 m2 |
Order 18.61 square metres. If the wall tile box covers 1.2 square metres, the wall portion of about 13.1 square metres including waste is 11 boxes. Keep wall and floor tiles as separate calculations, since they are almost always different products with different coverage per box. Put your own measurements into the tile calculator to skip the arithmetic.
When 10 percent is not enough
The waste allowance is not a formality. It covers tiles that break on the cutter, cuts that go wrong, and the offcuts that are the wrong shape for anywhere else in the room. Increase it when:
- The layout is diagonal or herringbone. Every edge of the room becomes a cut, and the offcut from one is rarely usable at the next. Use 15 percent, and 20 for herringbone with large tiles.
- The tiles are large format. A 600 x 1200 mm tile that cracks costs three times as much material as a 200 x 250 mm one.
- The room is not square. Older houses rarely are. An out-of-square wall means a tapered cut down one edge, and the tapered offcuts are waste.
- The tile has a pattern or directional grain. If it has to line up or run one way, you lose the ability to rotate an offcut to make it fit.
Do not forget the adhesive and grout
Tiles are the headline number, but the job needs more. As a rough guide, a 20 kg bag of tile adhesive covers around 4 to 5 square metres at a typical bed thickness, and less under large-format tiles which need a thicker bed. Grout is far less: 5 kg is often enough for 10 square metres of small wall tiles, though wide joints and large tiles change that considerably.
Both are on the tile suppliers' own coverage charts, and both depend on trowel size and how flat the substrate is. A wall that needs building out will drink adhesive.
The order of operations
- Measure every tiled surface separately, in metres.
- Multiply out each one and add them up.
- Subtract only the large openings.
- Add 10 percent, or 15 for diagonal, large-format or awkward rooms.
- Split the total into wall tiles and floor tiles.
- Divide each by the coverage per box and round up.
- Check every box is the same batch before you start.
For the rest of a bathroom refit, the paint calculator handles the ceiling and any untiled walls, and the Calc Garden guides cover the other measure-up jobs around the house.