Grout Calculator
| Tile + joint | 100 sq ft | 200 sq ft | 400 sq ft |
|---|---|---|---|
| 12-in tiles, 1/8-in joints | ~9 lb | ~18 lb | ~36 lb |
| 6-in tiles, 1/8-in joints | ~15 lb | ~30 lb | ~60 lb |
| 4-in tiles, 1/8-in joints | ~22 lb | ~44 lb | ~88 lb |
Grout math is the joints, not the tiles: tile length and width times the joint width, divided by the tile area, times the square footage - the formula every bag prints, and the reason a wall of 4-inch tiles drinks three times the grout of 12-inch tiles at the same square footage.
Bottom line: a 100-square-foot bathroom floor of 12-inch tiles with 1/8-inch joints wants about 8 pounds of grout, while the same floor in 4-inch tiles wants 20 - and the 10 percent waste rule on top covers the mixing losses, the pot that stiffened, and the joint you over-packed at the doorway.
The honest part: the sanded-versus-unsanded fork decides the bag before the math does - sanded for joints an eighth of an inch and wider, unsanded for the hairlines under an eighth - and mixing whole bags matters because grout has a pot life: the batch that sits past 90 minutes goes in the trash, so the arithmetic sizes batches, not just the total.
How to use
- Enter the tile's face dimensions, the joint width, and the total square footage - the calculator runs the printed-bag formula per pound.
- Add the 10 percent waste margin, then divide by the bag size for the bags to buy - and buy them from one lot, because color drifts between lots.
- Mix by the pot life: batches small enough to use in 90 minutes, damp-cured per the bag, and sealed a week later so the grout lines stay the color you chose.
Frequently asked questions
How much grout do I need per square foot?
It depends on the tile: 12-inch tiles with 1/8-inch joints run about 0.08 pounds per square foot, 4-inch tiles about 0.2, and mosaics with their many joints can hit half a pound. The formula - tile width plus length, times joint width, times joint depth, divided by tile area - is what the bags print, and the calculator runs it before the 10 percent waste margin.
Should I use sanded or unsanded grout?
Sanded for joints an eighth of an inch and wider - the sand is the strength that stops wide joints from cracking - and unsanded for the hairlines under an eighth, where sand grains will not fit and scratch delicate glass or polished stone. The vertical shower walls want the same fork, and epoxy grout is the premium third option that prices triple and forgives everything.
How long does mixed grout last in the bucket?
About 90 minutes of working time - past that the pot life is spent, the grout stiffens, and adding water to revive it weakens the cured joint and causes the blotchy color called mottling. Mix half bags for small rooms, and mix the second batch only after the first is in the joints.
Why did my grout turn out blotchy or crack?
Blotches come from uneven curing - some joints damp-cured, some left to dry fast in the sun - and cracks come from too much water in the mix or in the cleanup sponge. The fix is damp-curing per the bag for three days with a fine mist, and sealing a week later so the color stays the color you mixed.