The geometry behind the number
Grout fills the joint lines, so the math is joint length × joint cross-section. A square foot of 12×12 tile hides about 24 inches of joint line; a square foot of 4×4 mosaic hides 72. That is why mosaics eat three times the grout of large-format tile at the same joint width. The calculator adds 15% waste — tight for straight lays, generous for mosaics.
Sanded or unsanded
Joints 1/8 inch and narrower take unsanded grout — sand grains cannot squeeze into a gap that tight. Wider joints take sanded, which resists cracking. Anything over 1/2 inch needs a high-performance or expansion-joint product instead of more grout.
Depth is part of the math
Grout fills the joint from the tile face down to the substrate — on ⅜-inch-thick floor tile that is a ⅜-deep channel unless you back-butter. Deeper joints scale grout use linearly, so a half-depth back-butter nearly halves the number. Measure the real tile thickness, don't guess it.