Grout fills the joints between tiles, so the amount depends on how many joints there are, how wide they are and how deep the tile is. Enter the area, the tile size, the joint width and the tile thickness to get the volume and an estimate in pounds and bags.
Grout formula
Joint length per sq ft (in) = 144 × (tile length + tile width) ÷ (tile length × tile width) Grout volume (cu in) = area × joint length per sq ft × joint width × tile thickness × (1 + waste %) Pounds ≈ volume (cu ft) × grout weight (lb per cu ft)
Worked example: 100 sq ft of 12 × 12 in tile
- Joint length per sq ft: 144 × 24 ÷ 144 = 24 in.
- Volume: 100 × 24 × 0.125 × 0.375 = 112.5 cu in, plus 10% = 123.8 cu in.
- At 110 lb per cu ft: 123.8 ÷ 1,728 × 110 = 7.9 lb, so one 25 lb bag.
Grout by tile size (100 sq ft, 3/8 in thick tile, 10% waste)
| Tile | Joint | Grout (lb) |
|---|---|---|
| 2 × 2 in | 1/16 in | 23.6 |
| 3 × 6 in | 1/16 in | 11.8 |
| 4 × 4 in | 1/8 in | 23.6 |
| 6 × 6 in | 1/8 in | 15.8 |
| 12 × 12 in | 1/8 in | 7.9 |
| 12 × 24 in | 1/8 in | 5.9 |
| 24 × 24 in | 3/16 in | 5.9 |
About the weight estimate
The volume is plain geometry, but the weight of mixed grout varies by product. The 110 lb per cu ft default is an estimate; grout makers print coverage charts by tile and joint size on the bag, and those charts should decide how much you buy. Small mosaic tiles have far more joint length per square foot, so they use much more grout than large-format tile.
Questions
How much grout for 100 sq ft?
About 7.9 lb for 12 × 12 in tile with a 1/8 in joint, but 23.6 lb for 2 × 2 in mosaics with a 1/16 in joint.
Sanded or unsanded?
Unsanded grout is generally used for joints of 1/8 in or less and sanded grout for wider joints; follow the grout maker’s guidance.
Why does tile thickness matter?
Grout fills the joint to nearly the full depth of the tile, so thicker tile needs more.
Sources
- NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Cubic inch, cubic foot and pound definitions.