Tile Calculator

Adhesive is specified by the notch trowel, and the trowel is chosen by tile format. A 100 × 100 mm mosaic bed needs about 3 kg/m², a 300 × 300 mm tile about 4 kg/m², a 600 × 600 mm porcelain tile 5 kg/m², and a large-format 1200 mm tile 6 kg/m² or more because the bed has to be deep enough to compensate for the tile's own bowing. Two rooms of 12 m² can differ by a whole bag for that reason alone.

The same logic applies on walls. A 5 kg bag spreads the same distance on a wall as on a floor, but the amount behind each tile is dictated by the notch and by whether the substrate is flat enough for a thin bed. On an uneven wall, back-buttering adds another 10 to 20 %.

A 300 × 300 mm tile is 0.09 m² on its own, so eleven tiles cover a square metre before joints; with a 3 mm joint the grout lines add about 2 %, so ten and a half tiles plus the spare. For 600 × 600 mm the count halves and the relative waste per cut doubles: a room that is not a whole number of tiles in either direction needs 10 % spare rather than 5 %.

Keep one box unopened. Fitters return three years later for a cracked tile and find the colour has moved on, and a box of ten tiles costs less than retiling a patch. This is also the honest answer to the question of whether 5 % is enough: for a straight lay in a rectangular room yes, for anything else no.

Grout runs at about 0.7 kg/m² for narrow joints with small tiles and up to 1 kg/m² for wide joints with large ones, so a 20 m² floor needs roughly 15 to 20 kg — one bag, not two. Buying grout by the same area rule as adhesive is a common way to double the order.

Levelling clips, spacers and waterproofing membrane are separate lines that scale with perimeter and wet areas, not with floor area. In a bathroom the tanking alone can cost more than the adhesive, and it is the part that stops water reaching the screed, so it belongs in the estimate before the tiles are ordered.