Prefer coverage that matches the job
A coverage figure is useful only when its tile dimensions, joint width and filled depth resemble the installation. In the grout calculator, coverage means the area covered by one complete pack of the entered mass.
Understand the geometry alternative
The geometry method estimates joint volume from a repeating rectangular grid. Smaller tiles create more joint length per square metre. Wider or deeper joints increase the amount filled. The product coefficient turns the geometric estimate into mass and must come from suitable product information.
The filled joint depth is not automatically the full tile thickness. Adhesive position, tile edges and the intended finish affect the space receiving grout. Check the intended installation rather than copying the tile thickness uncritically.
Combine only matching products
Different tile sizes can be separate zones while using the same grout. Different grout products, colors or pack masses need separate estimates. The calculator pools mass across zones before rounding to packs.
For example, 2.64 kg required from a 2 kg pack means buying 4 kg. The 1.36 kg difference is purchase rounding, not another allowance. Keep the product label and your measurements with the exported estimate.