Grout is a volume, not a rate per tile
The common shortcut — one bag per so many tiles — is wrong in a way that matters, because it assumes every tile has the same amount of joint around it. It does not.
Grout fills a space with three dimensions: the length of joint, its width, and its depth. Work it that way and it is right for any tile:
- Joint length per square metre depends on tile size. For a tile L by W metres, it is 1/L + 1/W. A 300 mm square tile gives 3.33 + 3.33 = 6.67 metres of joint per square metre. A 50 mm mosaic gives 20 + 20 = 40 metres.
- Multiply by joint width and by tile thickness. Floor grout fills the full depth of the joint, and that depth is the tile's thickness.
- Multiply by density, about 1800 kg per cubic metre, for the weight.
The reason the shortcut fails is in those numbers. Mosaic has six times the joint length per square metre that a 300 mm tile has. Any rule expressed per tile, or per square metre without reference to tile size, will be badly wrong at one end of the range.
A worked figure
Ten square metres of 300 mm tiles, 8 mm thick, with 3 mm joints:
- Joint length: 10 × 6.67 = 66.7 m
- Volume: 66.7 × 0.003 × 0.008 = 0.0016 m³
- Weight: 0.0016 × 1800 = about 2.9 kg
Under three kilograms for ten square metres. The same ten square metres in 50 mm mosaic needs about 17 kg — six times as much, from the same floor area.
Sanded or unsanded
The choice follows the joint width, and it is not a preference:
- Unsanded: For joints up to about 3 mm. Smooth, and it will not scratch soft or polished surfaces.
- Sanded: For joints of 3 mm and wider. The sand resists the shrinkage that would crack a wide joint of neat cement.
Use unsanded in a wide joint and it will crack as it cures. Use sanded against polished marble or glass and you may scratch the face while working it in. Where a wide joint meets a scratchable tile, an epoxy grout solves both problems at a considerably higher price and with a much shorter working time.
Adhesive: the trowel notch decides coverage
Tile adhesive is not measured by area alone, because the quantity depends entirely on the notch size of the trowel — and the notch is chosen from the tile.
The rule is that bigger tiles need a bigger notch, because a large tile needs a thicker bed to accommodate any unevenness in the substrate and to achieve full contact behind the tile.
- Small tiles and mosaic: A fine notch, and a bag covers a large area.
- 300 mm tiles: A medium notch.
- 600 mm and larger: A large notch, and coverage per bag falls sharply — often to half that of a small tile.
For a rectangular tile the notch follows the **longest** side. A 600 × 300 plank is a 600 mm job, not a 300 mm one.
The coverage rule that matters more than the estimate
For floors, and for anything wet, the adhesive must achieve near-full contact behind the tile — no voids. A tile bedded on five dabs has five supported points and a large unsupported area that will crack the first time something heavy lands on it.
Lift one tile early in the job and look at the back. If the ridges have not collapsed into a solid bed, the notch is too small, the adhesive has skinned over, or it is not being pressed home. That thirty-second check has saved more floors than any calculation.