Divides the area to be covered by the space one tile occupies, grout joint included. Dividing by the tile size alone, without adding the joint, overestimates the count. An allowance is added for tiles cut at the edges.
The number of tiles needed is the area to be covered divided by the space each tile occupies.
and are the tile dimensions and the joint width. Adding the joint in the denominator is the point: dividing by the tile size alone will not do.
Tiles do not sit edge to edge; a joint runs between them. A 300 mm tile with a 3 mm joint occupies a 303 mm square, or cm², against the cm² of the tile itself, 2% more. Overlook that 2% and the count comes out too high.
A floor of 3.0 by 2.4 m in 300 mm tiles with 3 mm joints. The area is 7.2 m² and each tile occupies 0.0918 m², so , rounded up to 79 tiles, or 10.9 per square metre. Allowing 5% for cuts at the edges brings it to 83.
Tiles at the edges have to be cut, and where the offcut cannot be used elsewhere, extras are needed. Allow more for patterned or large-format tiles.
This divides by area alone and takes no account of the setting out. Where joints have to line up, the count along each direction has to be decided separately.