The short version
tiles = area to cover ÷ area of one tile including its grout line, then add waste.
A tile takes up its own size plus one grout joint, which is why counting tiles by their nominal size always comes out short.
A worked example
A floor 3.6 m by 2.4 m, tiled with 300 × 600 mm tiles and 3 mm joints.
- Area: 3.6 × 2.4 = 8.64 m².
- One tile with its joint: 0.303 × 0.603 = 0.1827 m².
- 8.64 ÷ 0.1827 = 47.3, so 48 tiles to cover the floor.
- Add 10% for cuts: 48 × 1.1 = 52.8, so 53 tiles.
- At 8 tiles a box that is 7 boxes, or 56 tiles.
How much waste to allow
- 10% for a simple rectangular room laid straight.
- 15% for a diagonal layout, a herringbone pattern, or a room with an awkward shape.
- 15% or more for large-format tiles, where one bad cut wastes a lot of tile, and for any room with pipes, alcoves or a bay window.
- 5% is only enough for a plain square area with no obstacles and a tiler who does not break anything.
Waste is not just breakages. Most of it is the off-cuts at the edges, where the piece left over is too narrow to use anywhere else.
Buy a box more than you need
Tiles are made in batches, and the color shifts slightly between them. A box kept in the garage is the only way to replace a cracked tile in three years' time and have it match. Keep the batch number too.
Walls, splashbacks and areas you skip
Work out each wall separately, because they are rarely the same size, and subtract anything you are not tiling: a window, a door opening, a bath panel. Small gaps are better left in the total, since a tile cut around a small window still comes out of a whole tile.
Adhesive and grout
Coverage depends on the trowel notch and how flat the wall is, so follow the bag. As a rough guide, a 20 kg bag of adhesive covers 4 to 5 m² at 6 mm, and 5 kg of grout fills the joints of roughly 20 m² of 300 × 600 tiles at 3 mm.
Tools
The Tile Calculator does the arithmetic above, including the grout line and the area you are leaving out. The Paint Calculator covers the walls you are not tiling, and the Roof Area Calculator handles a sloping surface.