How many boxes of tiles for a 4.2 m by 3.6 m floor?

The floor is 15.12 m², 16.632 m² with 10% extra for cuts. In boxes that cover 1.44 m², 16.632 ÷ 1.44 = 11.55, so 12 boxes.

How much wall do I tile if the tiles only go up to 1.2 m?

The distance round the room times 1.2 m, less the doors up to 1.2 m. In a 4.2 m × 3.6 m room with one 826 mm door that is 15.6 × 1.2 = 18.72 m², less 0.99 m² for the door: 17.73 m².

Do I take the window off when tiling to half height?

Not unless you know where the sill is. A window that starts above the tiles takes nothing off them. BuildBatch takes windows off only when the walls are tiled to the ceiling, and says so.

How many tiles do I need for a floor or a wall?

Work out the area to tile in square metres, add an allowance for cuts, divide by what one box covers and round up to whole boxes. A 4.2 m × 3.6 m floor is 15.12 m², 16.632 m² with 10% extra, and needs 12 boxes that each cover 1.44 m².

A floor

Floor area = length × width. For an L-shaped room, split it into rectangles, or draw it in the room planner.
Add the allowance for cuts: 10% is a starting figure for tiles laid straight. A diagonal or herringbone layout, or large tiles in a small room, waste more; the tile’s fitting guide says how much.
Boxes = the area with the allowance ÷ what one box covers, from the box.
Round up to a whole box.

A wall

The area of wall to tile is the distance round the room times the height the tiles go up to.

Doors come off up to that height: the door’s width times the lower of the door’s height and the tiled height.
Windows come off only when the tiles go to the ceiling. Below that, a window may start above the tiles and take nothing off them; without the sill height, nothing is taken off.

Worked examples

A room 4.2 m × 3.6 m, 2.4 m high, one door 826 × 1981 mm and one window 1200 × 1100 mm, tiles in boxes that cover 1.44 m², 10% extra.

Tiling	Area	With 10%	Boxes of 1.44 m²
The floor	15.12 m²	16.632 m²	12
The walls to 1.2 m	18.72 − 0.99 = 17.73 m²	19.50 m²	14
The walls to the ceiling	37.44 − 1.64 − 1.32 = 34.48 m²	37.93 m²	27

To the ceiling, the door comes off at its full height (1.64 m²) and so does the window (1.32 m²).

Adhesive and grout

Both are sold in bags, and both bags say how far they go, which depends on the tile:

Adhesive: the bag gives the area it covers for the size of notched trowel, and the tile size decides the trowel. Floors on timber take a flexible adhesive.
Grout: the bag gives the area it covers for the tile size and the joint width.

Enter those figures in the planner and it counts the bags; without them, the line says what to check.

What goes under the tiles

A timber floor flexes, and tiles crack on a floor that moves: they go on tile backer board or overlay ply, screwed down.
A concrete floor or screed must be dry and flat: a damp floor is dealt with first, not covered over, and an uneven one is levelled.
A wall must be flat and sound: old wallpaper off, loose plaster made good, gloss paint sanded or scored. The adhesive says what it will stick to and whether plaster needs priming.
Round a shower or over a bath, the wall is waterproofed (tanked) before tiling. Grout and tiles are not waterproof on their own.

Before you buy

Buy every box from the same batch: the shade can differ between batches, and the box shows the batch.
Keep a few tiles back for a repair later.
Silicone, not grout, goes where tiles meet a bath, shower tray, basin or worktop, and round the edge of a tiled floor.

The room refresh planner works all of this out from one set of measurements: choose “Tile them” for the walls, or “Floor tiles” for the floor, and copy the figures from the box.