How the grout is worked out
The joint length in a square metre depends on the tile size. Small tiles mean more joints, which is why a mosaic needs far more grout than a large format tile.
The formula is the tile length plus the width, divided by the tile length times the width, multiplied by the joint width, the joint depth and the density of the grout.
Cementitious grout is about 1.6 grams per cubic centimetre. A 300 mm tile with a 3 mm joint 8 mm deep needs 0.256 kg per square metre.
Worked example
Twelve square metres of 300 mm tiles with a 3 mm joint 8 mm deep needs 0.256 kg per square metre, which is 3.07 kg in total.
The same area in 100 mm tiles needs about 0.9 kg per square metre, which is 10.8 kg. Small tiles use three times the grout.
Joint width and depth
The depth is the tile thickness less the adhesive bed, and it is the number people get wrong. A 10 mm tile on a 6 mm bed leaves a 4 mm deep joint, not 10.
Wider joints are not just a style choice. A 10 mm joint on a large format tile is structural, because it absorbs movement that would otherwise crack the tile.
Epoxy grout is denser than cementitious grout and covers less per kilogram, so use the manufacturer's figure for it.
Limits of this calculator
It assumes a rectangular tile and a full joint. A mosaic sheet, a herringbone pattern or a tile with a chamfered edge changes the joint volume.
It uses a density of 1.6 for cementitious grout. Epoxy, and grouts with a light aggregate, differ.
It does not include waste, which for grout is usually covered by buying a whole bag.
For the tiles themselves, use the tile calculator.
Inputs for the grout calculator
Tiled area
Area already tiled or about to be tiled, in square metres.
Tile length
Long edge of one tile in millimetres.
Tile width
Short edge of one tile in millimetres.
Joint width
Width of the gap between tiles, usually 2 to 5 mm.
Joint depth
How deep the joint is filled, which is the tile thickness less the adhesive bed.
CALCULATOR QUESTIONS
Grout calculator questions
How much grout per square metre?
About 0.26 kg for 300 mm tiles with a 3 mm joint, and about 0.9 kg for 100 mm tiles.
Should I use the tile thickness as the depth?
No. Subtract the adhesive bed, which is usually 3 to 6 mm.
Does a wider joint use more grout?
Yes, in direct proportion. Doubling the joint width doubles the grout.
Is epoxy the same?
No. Epoxy grout is denser and covers less per kilogram, so use the manufacturer's coverage.