How the grout calculator works
Grout fills the joints between tiles, all the way down to the thinset. So the amount depends on how much joint there is per square foot: small tiles, wide joints and thick tiles all need more. The calculator treats one tile plus its joint as a repeating block, works out the joint volume, and turns it into pounds or gallons.
Block = (tile length + joint) × (tile width + joint)
Joint volume per sq ft (cu in) = 144 × (block − tile length × tile width) × tile thickness ÷ block
Grout = area × joint volume × (1 + extra %)
Sanded: 0.065 lb per cu in. Non-sanded: 0.062 lb per cu in. Pre-mixed: about 220 cu in per gallon
The weights per cubic inch were matched to Custom Building Products' published coverage charts. For common tile sizes the calculator comes within about 2% of those charts; for small mosaics it errs slightly on the generous side.
Worked example
A 100 sq ft bathroom floor uses 12 by 12 in tile, 3/8 in thick, with 1/8 in joints. Each block is 12 1/8 by 12 1/8 in, so the joints hold 1.11 cubic inches per sq ft. That is 111 cubic inches, plus 10% is 122 cubic inches, or 7.9 lb of sanded grout. That is just over a 7 lb bag, so buy one 25 lb bag. In non-sanded grout with 1/16 in joints it is 3.8 lb, one 10 lb box; in pre-mixed grout with 1/8 in joints it is 0.55 gallons, one gallon pail.
How far a bag goes
Square feet covered by one 25 lb bag of sanded grout, from Custom Building Products' Polyblend Plus chart:
| Tile | 1/8 in joint | 3/16 in joint | 1/4 in joint |
|---|---|---|---|
| 2 x 2 in mosaic, 1/4 in thick | 96 | 68 | 54 |
| 6 x 6 in, 1/4 in thick | 266 | 181 | 139 |
| 12 x 12 in, 3/8 in thick | 348 | 234 | 177 |
| 12 x 24 in, 3/8 in thick | 461 | 310 | 234 |
| 24 x 24 in, 3/8 in thick | 688 | 461 | 348 |
Choosing a grout
| Type | Joints | Sold as | Notes |
|---|---|---|---|
| Sanded | 1/8 to 1/2 in | 25 lb and 7 lb bags | Strong and cheap; sand can scratch polished stone and glass |
| Non-sanded | Up to 1/8 in | 10 lb boxes | Smooth; can shrink and crack in wide joints |
| Pre-mixed | 1/16 to 1/2 in | 1 gallon and 1 quart pails | No mixing and usually no sealer; costs more per square foot |
Joint ranges are from Custom Building Products' Polyblend Plus and SimpleGrout data. Other brands come in other sizes, such as 8 lb and 25 lb bags; their charts give similar coverage per pound.
Before you buy
- Measure a joint after setting the tile; it is the spacer size only if you used spacers throughout.
- Clean thinset out of the joints before it hardens so the grout can fill the full depth.
- Buy every bag in one color batch, and mix only what you can use in about 30 minutes.
- Seal cement grout after it cures if the maker recommends it, especially in showers and kitchens.
Sources
Common questions
How much grout do I need for 100 square feet?
For 12 by 12 inch tile, 3/8 inch thick, with 1/8 inch joints, about 7 lb of sanded grout plus 10% extra, so 8 lb. One 25 lb bag covers about 348 sq ft of that tile on Custom Building Products' chart. Smaller tiles and wider joints need much more.
Why do small tiles need so much more grout?
Grout fills the joints, and small tiles have many more joints per square foot. On Custom Building Products' chart, a 25 lb bag covers 96 sq ft of 2 by 2 inch mosaic with 1/8 inch joints but 348 sq ft of 12 by 12 inch tile with the same joint: more than three and a half times as much grout for the mosaic.
Should I use sanded or non-sanded grout?
Use sanded grout for joints 1/8 inch and wider and non-sanded grout for joints of 1/8 inch or less. Non-sanded grout is also safer on polished marble and glass tile, which sand can scratch. Check the tile maker's advice.
How deep does grout go?
The full depth of the tile, down to the thinset. Scrape out any thinset that squeezes up into the joints before it sets so the grout can fill them.
How many square feet does a 25 lb bag of grout cover?
Anywhere from about 25 to 700 sq ft depending on tile size and joint width. Custom Building Products lists 348 sq ft for 12 by 12 inch tile with 1/8 inch joints and 54 sq ft for 1 by 1 inch mosaic with the same joint.