Grout calculator
Grout usage rises with joint width and joint depth, and falls as tiles get bigger. For a rough figure: multiply tile length plus tile width, divide by tile length times tile width, then multiply by joint width, joint depth and 1.6. On 30 cm tiles with a 3 mm joint and 8 mm depth that lands near half a kilogram per square metre — so a 12 m² floor takes a 5 kg bag with some to spare.
Grout use depends on joint width, joint depth and tile size, not on area alone. A 30 cm tile with a 3 mm joint takes roughly 0.5 kg per square metre; a small mosaic with the same joint can take four times that, because there is far more joint per square metre.
How to estimate grout
The reason a grout calculator cannot be a single multiplication is that the joint is a three-dimensional trench, not a line. Its volume is the length of joint per square metre, times the width, times the depth of the tile. Halving the tile size doubles the joint length. Doubling the joint width doubles it again. And a 20 mm porcelain plank has three times the joint depth of a 6 mm ceramic wall tile. Manufacturers publish coverage tables for exactly this reason, and the figure on the bag is worth trusting over any generic rule.
Questions
About 0.5 kg for 30 cm tiles at a 3 mm joint. Mosaics can need 2 kg or more, large-format porcelain as little as 0.2 kg.