Short definition
The kilograms of grout needed per square metre of tiling — set by tile size, joint width and tile thickness, not by area alone.
Grout coverage is the mass of grout needed to fill the joints in one square metre of tiling. It is not a property of the area being tiled: it falls out of the joint volume, which depends on the tile size, the joint width and the tile thickness. Grout fills the gap between tiles for the full depth of the tile, so a thicker tile needs proportionally more grout at the same joint width.
The relationship with tile size is the one that surprises people. A square metre of 300 × 300 mm tiles contains far more joint length than a square metre of 600 × 600 mm tiles, so it uses roughly twice the grout at the same joint width. Mosaics are the extreme case and can use several times what a large-format floor uses. Halving the joint width roughly halves the grout.
The standard calculation is joint volume per square metre multiplied by the density of mixed grout. Per m², that volume is ((tile length + tile width) ÷ (tile length × tile width)) × joint width × tile thickness, with every dimension in the same unit. Multiplied by density, it gives kilograms. As a worked example, 300 × 300 × 8 mm tiles with a 3 mm joint come out at about 0.34 kg/m² before any allowance for waste.
Two adjustments matter on a real order. Manufacturers publish coverage tables that already reflect their own product density, so those tables beat any generic formula when you have them. And mixed grout is lost to the bucket, the float and the sponge, so an allowance on top of the theoretical figure — around 15% — is what keeps a job from running a bag short. Grout is sold in fixed bag sizes, so the order rounds up to whole bags — the tile and grout estimator does that rounding from your tile size and joint width.