How much grout you need depends on the tile, not on the room. A 25 pound bag of grout covers 146 square feet of 3 inch tile and 776 square feet of 24 inch tile at the same joint width, because grout does not fill the floor. It fills the gaps between the pieces, and small tiles have far more gap per square foot than big ones.
Grout is bought by the joint
Lay out a square foot of tile and add up the joint running through it. With 12 inch tiles you get 24 inches of joint. With 6 inch tiles you get 48. With 3 inch tiles, 96. With one inch mosaic, 288. The floor has not changed size once.
Joint per square foot
( length + width ) × 144 ÷ ( length × width )
Tile length and width in inches · gives inches of joint in one square foot · multiply by the joint width and the tile thickness for the volume to fill
That is the whole mechanism. Everything else on a grout bag is a consequence of it.
A manufacturer’s own chart
Square feet covered by one 25 pound bag, from Mapei’s data sheet for Ultracolor Plus FA:
| Tile | 1/16 in joint | 1/8 in joint | 1/4 in joint |
|---|---|---|---|
| 3 x 3 x 1/4 in | 291 | 146 | 73 |
| 6 x 6 x 1/4 in | 582 | 291 | 146 |
| 12 x 12 x 3/8 in | 776 | 388 | 194 |
| 24 x 24 x 3/8 in | 1,552 | 776 | 388 |
Read it in either direction and it halves. Double the tile and the coverage doubles. Double the joint width and the coverage halves. Nothing about the room enters into it.
The whole chart is one number
Take any cell in that table, work out the joint volume in a square foot of that tile, and multiply. A 12 by 12 tile at an eighth of an inch gives 24 inches of joint, three eighths deep, an eighth wide: 1.125 cubic inches per square foot. Times 388 square feet is 437 cubic inches.
Do it for the 3 inch tile and you get 438. For the 24 inch tile, 437. For every other cell in the published chart, 437. The coverage table is not a table at all. It is one fact: a 25 pound bag of this grout fills about 437 cubic inches of joint, and a 10 pound bag fills about 175.
That is worth knowing because your tile is probably not on the chart. Work out the joint volume for the tile you actually bought, divide 437 into it, and you have the answer for a size no manufacturer bothered to print.
How much grout you need on a real floor
An 80 square foot bathroom floor, eighth inch joints, in 10 pound bags:
- 24 inch tile: about a quarter of a bag
- 12 inch tile: about half a bag
- 6 inch tile: two thirds of a bag
- 3 inch tile: one and a half bags, so two
- 1 inch mosaic: four bags
Same room, same joint, same grout, and the order runs from a quarter of a bag to four. If you have ever wondered why the tile shop asks what size tile before they will tell you how much grout to buy, that is why.
The three things that move the number
Tile size is the big one, and it is decided before you get anywhere near the grout aisle. The other two are worth a moment.
Joint width is linear. Going from an eighth to a quarter doubles the grout, and it is a choice people make on the day for looks. Tile thickness is the depth of the joint, so a thicker tile holds more grout at the same width. That is why the chart pairs 3 eighths thick with the large formats and a quarter with the small ones: it is not a detail, it is a third of the calculation.
Buy the round up, not the exact figure
Grout comes in whole bags and the arithmetic above is a minimum. Mapei prints the usual caveat on its own chart, that coverage varies with the real tile size, the real joint width and how the job is grouted, and that is fair: joints are never exactly the width you planned, and a lippage-heavy floor swallows more.
Round up and buy the whole job in one go, for the same reason as the tile itself. Cementitious grout is pigmented in batches and two bags from different runs can dry to visibly different shades, which on a grid of joints is the most visible defect a tiled floor can have.
Sealer is a separate order and is sold by the square foot of floor rather than by the joint, so nothing above applies to it.
Work out the tile first
Frequently asked
It depends on the tile size, not the room. One 25 pound bag covers about 146 square feet of 3 inch tile, 388 square feet of 12 inch tile and 776 of 24 inch tile, all at eighth inch joints, because grout fills the joints rather than the floor.
Work out the joint in a square foot first: tile length plus width, times 144, divided by length times width. A 12 inch tile gives 24 inches of joint per square foot, a 6 inch tile 48 and a 3 inch tile 96. Multiply that by the joint width and the tile thickness for the volume to fill.
There is no single figure, but there is a single constant behind every published chart. A 25 pound bag fills roughly 437 cubic inches of joint and a 10 pound bag about 175, whatever the tile. Divide that by the joint volume in one square foot of your tile and you have the coverage.
Directly and in proportion. Going from an eighth of an inch to a quarter doubles the grout on the same floor. Tile thickness does the same thing in the other dimension, since it sets how deep each joint is.
Yes. Cementitious grout is pigmented in batches and two bags from different runs can dry to visibly different shades. On a floor that is a grid of joints across the whole room, a shade change is the most visible defect you can get, so buy the whole job at once and round up.