How this calculator works. Grout uses the standard industry volume formula: (tile length + width) ÷ (length × width) × joint width × tile thickness, converted to pounds at typical grout density and divided into 25-lb bags with a 10% margin. This is the same formula behind manufacturer coverage charts — which is why our numbers track the Custom Building Products and Mapei tables.
Thinset is matched to the trowel your tile size requires: 1/4" square notch for mosaics and subway (≈90 sq ft per 50-lb bag), 1/4×3/8" for 12"-range tile (≈65 sq ft), 1/2" for large-format (≈45 sq ft) — plus back-buttering allowance on 24" tile.
Grout and thinset, sized honestly
These two bags are where tile-day emergencies happen: grout math is genuinely unintuitive (small tiles need far more grout than large ones — a subway-tile shower has several times the joint length per square foot of a 24-inch floor), and thinset runs out because the trowel was bigger than the estimate assumed. Compute both from your actual tile and joint, not a generic per-square-foot rule.
One warning that saves real money: sanded grout for joints 1/8" and wider, unsanded for narrower joints and polished stone (sand scratches it). Buying the wrong type isn't a small mistake — unsanded grout in wide joints cracks and crumbles within months.
Trowel size by tile
| Tile | Trowel | ≈Coverage per 50-lb thinset |
|---|---|---|
| Mosaic, subway (≤8") | 1/4" square | 90 sq ft |
| 12"-range tile | 1/4×3/8" | 65 sq ft |
| Large format (≥15" any side) | 1/2" + back-butter | 40–45 sq ft |
Buying tips
Use unmodified thinset over membrane systems (check the membrane's spec) and modified everywhere else; large-format tile wants an LHT (large-and-heavy-tile) mortar that resists slump. For grout color, buy all bags from one lot and dry-mix them together — grout pigment varies between batches just enough to see.
Common questions
Sanded or unsanded grout?
Joint width decides: 1/8" and wider → sanded; narrower → unsanded. Exception: polished marble and soft stone always get unsanded (or epoxy), since sand scratches the surface at the joint edge.
What about epoxy grout?
Stainproof and waterproof, roughly 3× the cost, and less forgiving to install (short working time, thorough haze cleanup). Worth it in showers and kitchen backsplashes; overkill on a bedroom floor.
How thick should thinset be?
After combing and setting the tile, aim for 3/32"–1/8" of continuous coverage — check by pulling a tile up early in the job. Voids under large tile crack corners; that's what back-buttering prevents.
Can I grout the same day I set tile?
Wait until the thinset cures enough to lock tiles — typically 24 hours (some rapid-set mortars allow same-day; the bag says). Grouting too early shifts tiles and traps moisture.