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

$R = \left\lceil \frac{L + g}{l + g} \right\rceil$

$C = \left\lceil \frac{W + g}{w + g} \right\rceil$

where R is the number of tiles in a row and C the number in a column, L and W are the length and width of the area, l and w the length and width of one tile, and g the gap size.

The ceiling brackets mean 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

$H = (C - 1) \times L$

$V = (R - 1) \times W$

$T = H + V$

where H is the combined length of the horizontal grout lines, V the combined length of the vertical ones and T the two added together.

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: Find the grout volume

$X = (R - 1) \times (C - 1)$

$G = (T \times g - X \times g \times g) \times t$

where X is the number of crossings, G the grout volume and t the tile thickness.

This treats each grout line as a long thin box: its length times its width (the gap size) times its depth (the tile thickness). The horizontal and vertical lines cross each other. At every crossing the two lines share one square of grout, one gap wide and one gap long. Subtracting one such square per crossing stops that grout being counted twice.

Step 4: Find the grout weight

$M = G \times d$

where M is the grout weight and d the grout density.

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 = 0.0017916 m³, which is 1.79 liters
• Grout weight: 0.0017916 × 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. Most residential tile jobs use a gap of 2 to 5 mm.
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.

Add 10 to 15 percent to the calculated grout weight to allow for mixing waste and future repairs, and buy that extra grout from the same batch so the color matches.

Grout density by type

The calculator's default density applies to standard cement-based grout. Other grout types are lighter or heavier, so the true weight of a different type can differ from the calculator's result. To get an exact weight for another grout, multiply the calculated volume by that product's density, usually listed on the packaging.

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? Yes. Buy 10 to 15 percent more than the calculated amount. This covers waste during mixing and application, and leaves extra 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. Doubling the gap from 2 mm to 4 mm nearly doubles the grout volume, for the same tiles and area. It falls a little short of double because the shared squares at the line crossings also grow when the gap grows.

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. Unsanded grout is generally used for gaps narrower than about 3 mm, and sanded grout for wider gaps, since the sand keeps a wide joint from 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.