How to calculate grout
Grout fills the joints, so the amount depends on how much joint there is: the tile size, the joint width and the joint depth, which is the tile's thickness. Small or thick tile and wide joints use a lot; big thin tile with a tight joint uses very little. The calculator works out the joint volume per square foot with the same module formula LATICRETE and MAPEI use, adds 10%, and converts it to pounds or pails for the product you pick.
Worked example — a 30 sq ft backsplash in 3×6 subway tile, 10 mm thick: with a 1/16 in joint the joints hold 52 cubic inches, 57 with 10% extra. Ultracolor Plus FA weighs about 0.058 lb per cubic inch of joint, so you need 3.3 lb — one 10-lb bag. Open the joint to 1/8 in and it doubles to 6.5 lb.
How far a 25-lb bag of sanded grout goes
| Tile | Joint | Lb per 100 sq ft | Sq ft per 25 lb |
|---|---|---|---|
| Mosaic (1 in chips) | 1/16 in | 33.2 | 75 |
| 3×6 subway | 1/8 in | 21.9 | 114 |
| 4×12 subway | 1/8 in | 11.8 | 211 |
| 4×4 4¼ wall | 1/16 in | 8.4 | 298 |
| 6×24 plank | 1/8 in | 7.7 | 325 |
| 12×12 ceramic | 1/8 in | 7.3 | 343 |
| 12×24 porcelain | 1/8 in | 5.2 | 479 |
| 24×24 porcelain | 1/8 in | 3.7 | 680 |
| 24×48 large format | 1/8 in | 1.9 | 1,321 |
MAPEI Keracolor S at the carton's suggested joint and the tile's real thickness, including 10% extra. MAPEI's own chart lists 383 sq ft per 25 lb for 12×12 tile with a 1/8 in joint, before any extra.
Picking the joint width
The joint is set by the tile, not by taste. Rectified porcelain, with machine-cut square edges, can go as tight as 1/16 in. Most ceramic and porcelain tile uses 1/8 in — the joint Floor & Decor suggests for its 3×6 subway tile. Pressed-edge tile with rounded, slightly uneven edges needs about 3/16 in, and rustic or handmade-look tile 1/4 in. Grout use scales almost directly with the joint: going from 1/16 to 1/8 in roughly doubles it, which is why the table in the calculator shows every common width. Measure the joint you will actually leave with your spacers, and check the grout's rated range before you buy.
Sanded, unsanded, pre-mixed or epoxy?
- Unsanded is for joints up to 1/8 in, mostly wall tile. It shrinks and cracks in wider joints.
- Sanded is for 1/8 in and wider — floors and most porcelain. Fine-aggregate grouts like Ultracolor Plus FA cover both ranges and are safe on glossy or soft tile.
- Pre-mixed (Fusion Pro, Flexcolor CQ, LATICRETE RTU) comes in pails of about 220–231 cubic inches of joint per gallon and never needs sealing. One gallon of Fusion Pro covers about 248 sq ft of 12×24 tile at a 1/8 in joint.
- Epoxy (Kerapoxy CQ, SPECTRALOCK PRO) is the most stain-proof choice for showers and kitchens. Check the joint limit — Kerapoxy CQ is rated only for 1/16–3/8 in joints.
Common mistakes
- Ignoring tile thickness. A 10 mm subway tile needs about 57% more grout than the 1/4 in tile in most published charts.
- Underestimating mosaics. Penny and 1-in mosaics take about 33 lb of sanded grout per 100 sq ft, many times what large tile uses.
- Buying by weight across brands. A 17-lb box of Prism covers what a 25-lb bag of standard grout does.
- Throwing out the leftovers. Keep a little powder from the same batch so repairs match.
100 sq ft of 12×24 at 1/8 in takes only about 5.2 lb, so for large tile a small bag or a quart of pre-mixed grout is often enough. Count tiles and boxes with the tile calculator and setting mortar with the thinset calculator.