Of every number on a tile job, grout quantity varies the most. The same square footage can take 130 pounds or 1,100 depending on tile size and joint width — because grout fills joint volume, and small tile has vastly more joint per square foot.
Why the range is so wide
Joint volume is joint width times tile thickness times total joint length. Small tile has enormous joint length per square foot; large tile has almost none.
Doubling joint width roughly doubles the grout. Halving tile size roughly doubles it again. The two multiply.
Sanded or unsanded
Sanded grout goes in joints an eighth of an inch and wider. The sand keeps it from shrinking and cracking as it cures.
Unsanded is for joints narrower than an eighth — typically wall tile and polished stone, which sanded grout would scratch. Using unsanded in a wide floor joint produces cracks within a season.
Buy one bag more than the number
Grout is cheap and running out mid-floor is expensive. Colour matching between batches is imperfect, and a partially grouted floor cannot wait for a supply run.
Mix by full bag where you can. Adding water by eye to a half bag is the usual cause of grout that dries two different shades.
Mixing is where most grout jobs go wrong
Grout is mixed to a stiff, peanut-butter consistency and then left to stand — slaked — for five to ten minutes before a final stir. That rest lets the polymers wet out fully.
The temptation is to add water to make it easier to spread. Every splash of extra water weakens the cured grout and lightens the colour, and because you are mixing bag by bag, the batches end up different shades. On a large floor that shows as patches.
Mix by full bag wherever the job is big enough, measure the water rather than pouring by eye, and use the same ratio every time.
Sealing, and what it does not do
Cement grout is porous. A penetrating sealer slows down staining and makes cleaning easier, but it does not make grout waterproof and it does not stop it cracking.
Wait until the grout has fully cured before sealing — usually seventy-two hours, longer in a cold or humid room. Sealing damp grout traps the moisture and can leave a haze.
Epoxy grout is the alternative that genuinely does not need sealing. It costs several times as much, it is unforgiving to work with because it sets on a schedule of its own, and in a shower or a commercial kitchen it is worth both.