Grout is easy to under-buy because the bag sizes look generous and the joints look small. The right amount depends on four things: tile size, joint width, joint depth and the total area. Smaller tiles mean more joints, which is why a mosaic floor can take several times more grout than a large-format floor of the same size. Here is the math and a chart for common tile sizes.
The quick answer
For typical floor tile (12 x 12 inches) with 1/8 inch joints, plan on roughly 5 pounds of grout per 100 square feet (joints 1/4 inch deep). Smaller tiles and wider joints use more – a 3 x 6 subway tile with 1/8 inch joints needs about three times as much. The chart below covers common combinations.
Grout chart by tile size and joint width
Pounds of grout per 100 square feet. Every row assumes joints 1/4 inch deep, about the thickness of standard wall tile. Joint depth follows tile thickness, so for 3/8 inch thick tile multiply the figures by 1.5, and for 1/2 inch tile double them:
| Tile size | 1/8 inch joints | 1/4 inch joints |
|---|---|---|
| 24 x 24 inches | about 3 lb | about 5 lb |
| 12 x 12 inches | about 5 lb | about 10 lb |
| 6 x 6 inches | about 10 lb | about 21 lb |
| 3 x 6 inches (subway) | about 16 lb | about 31 lb |
| 1 x 1 inch mosaic | about 62 lb | about 125 lb |
These figures are worked out from the tile dimensions and the approximate density of cement grout (about 0.07 lb per cubic inch). Round up, because real joints are never perfectly uniform. Manufacturer coverage figures vary by product and by the tile thickness they assume, so treat the coverage chart on your bag or the manufacturer’s online calculator as the final word:
- MAPEI tile grout calculator – coverage by product, tile size and joint width
- MAPEI Ultracolor Plus FA technical data sheet – coverage tables per bag
- Custom Building Products Polyblend sanded grout technical data sheet – joint-width ratings and directions
The formula, if you want to check the math
For rectangular tiles (length L and width W in inches), the inches of grout joint in one square foot of tile are:
Joint inches per sq ft = 144 × (L + W) ÷ (L × W)
For a square tile that simplifies to 288 ÷ side. Multiply the joint inches by the joint width and depth (both in inches) to get cubic inches of grout per square foot, then multiply by 0.07 – the approximate weight in pounds of one cubic inch of cement grout – to get pounds per square foot.
Worked out for 12 x 12 tile with 1/8 inch wide, 1/4 inch deep joints: 288 ÷ 12 = 24 joint inches per square foot. 24 × 0.125 × 0.25 = 0.75 cubic inches. 0.75 × 0.07 ≈ 0.05 lb per square foot, or about 5 pounds per 100 square feet, matching the chart.
Worked examples
100 sq ft kitchen floor, 12 x 12 tile, 1/8 inch joints
From the chart: about 5 lb per 100 sq ft. One 10 lb bag covers the floor with plenty left for waste. If the tile is 3/8 inch thick rather than 1/4, the same floor needs about 8 lb.
60 sq ft subway tile backsplash, 3 x 6 tile, 1/8 inch joints
About 16 lb per 100 sq ft means 60 sq ft needs roughly 10 lb. One 10 lb bag just covers it – buy a second small bag or a 25 lb bag so you are not stretching the last cup of grout across the last square foot.
Shower floor in 1 inch mosaic, 12 sq ft
Mosaics are grout-hungry: about 62 lb per 100 sq ft means 12 sq ft needs roughly 7-8 lb. For a whole shower (floor plus walls) work each surface out separately and add them together.
Tips before you buy
- Add 10 to 15 percent on top of any estimate. Joint depth varies, some grout is lost in mixing and cleanup, and leftover grout is useful for future repairs.
- Sanded vs unsanded: the conventional rule is sanded grout for joints 1/8 inch and wider and unsanded for narrower joints and scratch-prone tile, but products vary – one major sanded grout is rated for joints 1/8 to 1/2 inch while its maker suggests unsanded up to 1/8 inch. Follow the joint-width rating on your chosen grout and the tile manufacturer’s compatibility directions. The chart above works for either type.
- Epoxy grout is different: it is sold as multi-part kits rated by coverage, so use the kit’s coverage figure instead of this chart.
- Mix in batches: mixed cement grout has a limited pot life – about 30 minutes to 1 hour for fast-setting products and 1 to 2 hours for standard cement grouts. Mix what you can use before it stiffens, not the whole bag at once.
Related calculators and guides
- Tile calculator – how many tiles the area needs
- How many tiles do I need for a floor
- Square footage calculator – measure the room first
- Flooring calculator – for other floor types
Estimates only. This is general guidance, not professional advice. Confirm details with your supplier, installer or local building department.