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Grout Calculator
Grout is the bag everyone under-buys. Enter your tiled area, tile size and joint width and this calculator works out the grout you need — in pounds, in 25 lb and 10 lb bags, and as a coverage figure you can check against the chart printed on any bag. It uses the actual joint geometry (formula below), calibrated to manufacturer coverage charts — not a flat "one bag per X square feet" guess.
Results update as you type. Estimates only — confirm final quantities before ordering.
The formula
Grout volume per ft² = Joint fraction × Joint depth × 144 in³
Weight = Volume × 0.0589 lb per in³
The joint fraction is the share of the floor that is grout line rather than tile — for 12 in tiles with 1/8 in joints it is about 2.05%. The constant 0.0589 lb per cubic inch is the yield density implied by MAPEI's published coverage data, and it is what makes this formula reproduce MAPEI's chart: 12×12 in tile with 1/8 in joints comes out at 383 ft² per 25 lb bag, exactly MAPEI's figure (SGM's chart says 392 for the same case). The formula assumes joints are cleaned out and filled flush to the full joint depth. Tile bevels, lippage and wash-out during cleaning are what the waste allowance is for — and for mosaics and very small tiles, whose manufacturer coverage runs lower than pure geometry, use a 15–20% allowance.
Worked example
Example: a 120 ft² bathroom floor, 12 in × 12 in tiles, 1/8 in joints, 3/8 in deep, 10% waste.
- Joint fraction: 1 − (12 × 12) ÷ (12.125 × 12.125) = 2.05% of the floor is grout line
- Volume per ft²: 0.0205 × 0.375 in × 144 = 1.11 in³ → weight 1.11 × 0.0589 = 0.065 lb per ft²
- Total: 120 × 0.065 = 7.8 lb; with 10% waste 8.6 lb → buy one 25 lb bag (or one 10 lb bag)
- Coverage check: 25 ÷ 0.065 = 383 ft² per 25 lb bag — the same figure MAPEI publishes for this tile and joint
Reference chart
| Tile size (joint depth) | 1/8 in joints | 3/16 in joints | 1/4 in joints |
|---|---|---|---|
| 3 × 6 in (1/4 in deep) | 198 | 135 | 104 |
| 6 × 6 in (1/4 in deep) | 292 | 198 | 150 |
| 8 × 8 in (3/8 in deep) | 257 | 174 | 132 |
| 12 × 12 in (3/8 in deep) | 383 | 257 | 195 |
| 12 × 24 in (3/8 in deep) | 509 | 341 | 258 |
| 24 × 24 in (3/8 in deep) | 760 | 509 | 383 |
Frequently asked questions
How much grout do I need for 100 sq ft of 12x12 tile?
With 1/8 in joints filled 3/8 in deep, about 6.5 lb of sanded grout — one 25 lb bag covers roughly 383 ft² at those settings, so a single bag handles most bathrooms with plenty left for repairs. Widen the joints to 1/4 in and the same floor needs about 12.8 lb.
Does joint width really change grout usage that much?
Yes — it is the biggest lever after tile size. On 12 in tile, going from 1/8 in to 1/4 in joints roughly doubles the grout (coverage falls from 383 to 195 ft² per bag), and small tiles multiply the joint length per square foot, which is why mosaics can need five times the grout of large-format tile.
Should I use sanded or unsanded grout?
Sanded grout for joints 1/8 in and wider — the sand stops shrinkage and cracking. Unsanded for joints under 1/8 in and for polished stone, glass and other scratch-sensitive tile. This calculator's coverage is calibrated to cement-based sanded grout; epoxy and pre-mixed grouts are sold by volume and carry their own coverage charts.
Why do manufacturer charts show less coverage for mosaics than the formula?
Charts such as MAPEI's build in real-world losses that pure geometry misses — beveled tile edges widen the joint at the surface, some grout washes out during cleaning, and mesh-mounted mosaics rarely sit at a perfectly uniform depth. The gap is small on 6 in and larger tiles (about 2%) but grows on 1–2 in mosaics, so raise the waste allowance to 15–20% there.
What should I enter for joint depth?
The tile's thickness is the standard assumption: grout is packed to the full depth of the joint. Common thicknesses are 1/4 in for wall and mosaic tile and 3/8 in for floor tile. If the thinset has squeezed up into the joints, the effective depth — and the grout needed — is less.
Related calculators
- Tile Calculator — Tiles and boxes needed, with waste for cuts and breakage.
- Paint Calculator — Gallons of paint for a room's walls, by coats.
- Concrete Calculator — Cubic yards and bag counts for slabs, patios and pads.
Sources & method: Joint-volume geometry (the standard grout-coverage method). Yield density calibrated to MAPEI's published Keracolor S coverage chart, fetched October 2026 (25 lb bag: 12×12 in tile, 1/8 in joint = 383 ft²; 24×24 in = 768 ft² — this formula gives 383 and 760). Cross-checked against SGM's grout coverage chart (392 ft² per 25 lb bag for 12×12 in at 1/8 in).