Grout Calculator: Volume and Weight for Tile Jobs

What is a grout calculator?

A grout calculator estimates how much grout a tiling job needs. Grout is the paste, usually made of cement, sand, and water, that fills the gaps between tiles. The calculator uses the size of the area being tiled, the size and thickness of the tiles, and the width of the gap between them to work out the total length of grout lines, the volume of grout that fills them, and the weight of grout to buy.

Why grout amounts matter

Buying too little grout means stopping partway through a job to mix a new batch, which can leave a slightly different color where the two batches meet. Buying too much wastes money, since mixed grout cannot be stored and dry grout has a shelf life. A calculation based on the actual dimensions of the room and the tiles avoids both problems.

How to calculate grout: the formula

The calculator works in five steps.

Step 1: Count the tiles in each row and column

1Tiles in Row = ceil((Area Length + Gap Size) ÷ (Tile Length + Gap Size))
2Tiles in Column = ceil((Area Width + Gap Size) ÷ (Tile Width + Gap Size))
3

"ceil" means round up to the next whole number. A tile that only partly fits at the edge of the room still has to be cut and laid, so it counts as a full tile, and it still needs grout around it.

Step 2: Find the length of the grout lines

1Horizontal Grout Length = (Tiles in Column − 1) × Area Length
2Vertical Grout Length = (Tiles in Row − 1) × Area Width
3Total Grout Length = Horizontal Grout Length + Vertical Grout Length
4

There is one grout line between each pair of neighboring tiles, so the number of lines in each direction is one less than the number of tiles.

Step 3: Count the crossings

1Crossings = (Tiles in Row − 1) × (Tiles in Column − 1)
2

Wherever a horizontal grout line crosses a vertical one, the two lines share a small square of grout, one gap wide on each side. Counting both lines in full would count each of these squares twice.

Step 4: Find the grout volume

1Grout Volume = (Total Grout Length × Gap Size − Crossings × Gap Size²) × Tile Thickness
2

This treats each grout line as a long thin box: its length times its width (the gap size) times its depth (the tile thickness). Subtracting one gap-sized square per crossing removes the double count.

Step 5: Find the grout weight

1Grout Weight = Grout Volume × Grout Density
2

The calculator uses a default density of 1600 kg per cubic meter (equal to 1.6 kg per liter), which is the typical density of mixed cement-based tile grout.

Worked example

Take a bathroom floor 3 meters long and 4 meters wide, tiled with 0.3 m square tiles that are 8 mm (0.008 m) thick, laid with a 3 mm (0.003 m) gap.

• Tiles in row: ceil((3 + 0.003) ÷ (0.3 + 0.003)) = ceil(9.91) = 10
• Tiles in column: ceil((4 + 0.003) ÷ (0.3 + 0.003)) = ceil(13.21) = 14
• Total tiles: 10 × 14 = 140
• Horizontal grout length: (14 − 1) × 3 = 39 m
• Vertical grout length: (10 − 1) × 4 = 36 m
• Total grout length: 39 + 36 = 75 m
• Crossings: (10 − 1) × (14 − 1) = 117
• Grout volume: (75 × 0.003 − 117 × 0.003²) × 0.008 = 0.223947 × 0.008 ≈ 0.00179 m³, which is about 1.79 liters
• Grout weight: 0.00179 × 1600 ≈ 2.87 kg

This floor needs about 140 tiles, 75 meters of grout line, and roughly 2.9 kg of grout.

How to use the calculator

1. Measure the length and width of the area to be tiled, in meters. Split an irregular room into rectangles and calculate each one separately.
2. Measure the length, width, and thickness of one tile, in meters.
3. Decide on a gap size. The right width depends on the tile. One U.S. federal tiling specification calls for 2 mm joints for glazed wall tile and mosaic, and 6 to 9 mm joints for quarry tile.
4. Enter all six values into the calculator.
5. Read off the number of tiles needed, the total grout line length, and the grout volume and weight.

The result has no allowance for waste. Many installers buy a little extra for mixing losses and future repairs, from the same batch so the color matches.

Grout density by type

The calculator's default density of 1600 kg/m³ (1.6 kg/L) applies to standard cement-based grout. Other grout types, such as epoxy, urethane, and furan grouts, have different densities, so their true weight can differ from the calculator's result. To get the weight for another grout, multiply the calculated volume by that product's density, usually listed on the packaging or data sheet.

1 kg/L equals 1000 kg/m³, the unit the calculator uses internally.

Frequently asked questions

Should I buy more grout than the calculator suggests? Usually, yes. The calculator gives the volume of the joints only. A small extra amount covers waste during mixing and application, and leaves grout from the same batch for later touch-ups.

Why does the calculator round the tile count up instead of down? Because a partial tile at the edge of a room still has to be cut and installed. There is no such thing as an unfinished edge in a real tiling job, so every partial tile counts as one whole tile, and it still forms a grout line.

Does a wider gap use more grout? Yes, and by a large amount. Grout volume is directly proportional to gap size, so doubling the gap from 2 mm to 4 mm roughly doubles the grout volume, for the same tiles and area.

Does tile thickness affect how much grout is needed? Yes. Grout volume is directly proportional to tile thickness, because the grout fills the gap for the full depth of the tile. Thicker tiles need proportionally more grout.

What is the difference between sanded and unsanded grout? Both use the same volume formula. A U.S. Department of Veterans Affairs tiling specification calls for unsanded grout in joints 3.2 mm (1/8 inch) and narrower, and sanded grout in joints 3.2 mm and wider. The sand helps a wide joint resist shrinking and cracking as it dries. Their densities are similar, so weight estimates apply to both.

Can the calculator be used for non-square tiles or unusual layouts? Yes. Enter the tile's length and width separately; they do not need to be equal. For irregular or hexagonal tiles, use the average length and width, or split the area into simpler rectangular sections and add the results together.

References

1. U.S. Department of Veterans Affairs, via Whole Building Design Guide. Section 09 30 13: Tiling. VA tiling specification 09 30 13 — unsanded grout for joints 3.2 mm (1/8 inch) and narrower, sanded for 3.2 mm and wider; joint widths of 2 mm for glazed wall tile and mosaic, 6 to 9 mm for quarry tile.