Grout is sold in bags and used by the joint. Small tiles with wide joints can need five times the grout of large-format tiles in the same room, which is why one bag is never the answer. Enter your tile and joint and get the real figure.
Notes
Where the volume comes from
Per square metre, the joints form a grid. The grout volume is the joint width times the joint depth times the total joint length, and the joint length depends entirely on tile size — which is why small tiles drink grout.
Joint depth is usually the tile thickness
Grout normally fills the full depth of the tile. If you are grouting over an uneven bed or the adhesive has squeezed up into the joints, the effective depth is less and you will use less.
Adhesive by notch
Trowel notch size sets the bed thickness, roughly: 6 mm notch about 3 kg per square metre, 10 mm about 5, 12 mm about 6.5. Back-buttering large tiles roughly doubles it. Manufacturer figures beat these.
What this does not do
It does not choose a grout or adhesive for your situation. Wet areas, underfloor heating, movement joints and porous tiles all change the specification.