How much grout do I need?
Grout fills the joints, so the amount depends on how much joint there is per square foot: narrow joints and big tiles use far less than wide joints and small tiles. The formula is area x (L + W) ÷ (L x W) x joint width x tile thickness, converted to pounds at about 105 lb per cubic foot.
Example: 120 sq ft of 12" x 24" tile, 3/8" thick, with 1/8" joints uses about 0.05 lb per sq ft — around 7 lb with 10% extra, so one 10 lb bag covers it.
Sanded or unsanded?
Unsanded grout is for joints narrower than 1/8"; sand gives wider joints the strength to resist cracking and shrinking. Most sanded grouts are rated up to 1/2" joints. For glass, polished marble or other scratch-prone tile, many installers use unsanded or a urethane or epoxy grout.
Minimum joint width
The tile installation standard ANSI A108.02 sets 1/16" as the narrowest joint, and asks for at least 1/8" (rectified) or 3/16" (calibrated) when tile 15" or longer is laid in an offset pattern.
Planning the whole floor? The tile calculator works out tile, thinset and grout together.