Grout Calculator
Estimate how much grout a tiling job needs from the tile dimensions, the joint width and depth, the tiled area and the grout type. The calculator finds the joint volume, converts it to the weight of dry grout mix, rounds it into whole bags, and can price the materials and labour. Switch between metric and imperial units.
Formula
Worked example
300 mm square tiles, 8 mm thick, with 3 mm joints over 10 m²: joint fraction = 1 - (0.3 × 0.3) / (0.303 × 0.303) = 1.97%. Volume = 0.0197 × 10 × 0.008 = 0.00158 m³ = 1.58 L. At 1.6 kg/L (sanded) that is about 2.5 kg; add 10% to 2.8 kg, so two 4.54 kg (10 lb) bags cover it with margin.
How the grout calculation works
Unlike paint or tile adhesive, grout does not cover an area, it fills the network of joints between the tiles, so the amount you need depends on how much joint there is. For a regular grid of rectangular tiles of length L and width W set with a joint of width g, the fraction of the surface taken up by joints is 1 minus (L × W) divided by ((L + g) × (W + g)). The calculator multiplies that joint fraction by the whole tiled area and the joint depth (the tile thickness) to get the volume of grout the joints will hold, then converts it to a weight to buy and rounds it up into whole bags.
Grout type, density and bags
Grout is sold by weight in bags, so the calculator converts joint volume to weight using a density that depends on the grout type. Unsanded grout is the lightest at roughly 1.5 kg per litre and suits narrow joints under 3 mm, sanded grout sits near 1.6 kg per litre for joints 3 mm and wider, and epoxy grout is around 1.7 kg per litre and is stain and water resistant. Choose your bag size and the calculator divides the total weight by the bag weight and rounds up, so you order whole bags. You can override the density under advanced options if your product lists a different figure, and the coverage output shows the grout used per square metre or square foot for quick cross-checks.
Why tile size and joint size matter so much
The same area of wall can need wildly different amounts of grout. Small tiles pack many more joints into each square metre, so a mosaic sheet of 25 mm tiles uses far more grout than a 600 mm porcelain slab, even with identical joint widths. Joint width and depth both multiply straight into the volume, so widening a joint from 2 mm to 4 mm roughly doubles the grout, and a thick floor tile fills more deeply than a thin wall tile. Because these factors compound, eyeballing the quantity is unreliable and the per-joint calculation is the dependable way to estimate.
Costing the job and good practice
Turn on the cost estimate to price the grout by the bag, and add a labour rate per area for a fitted total. Grouting labour commonly runs about 3 to 7 per square foot (roughly 30 to 75 per square metre), and epoxy grout costs several times more per bag than cementitious grout, so the type you pick drives much of the price. Allow about 10% extra for what is lost mixing, spreading and cleaning off the tile faces, and round up to whole bags. These figures are estimates: always cross-check against the coverage table printed on your chosen grout, which assumes a specific tile and joint size, and confirm local prices before you buy.
Grout type and joint guidance
| Grout type | Best for | Typical joint width | Approx. density |
|---|---|---|---|
| Unsanded (cementitious) | Walls, polished and soft tile | Under 3 mm (1/8 in) | 1.5 kg/L |
| Sanded (cementitious) | Floors and most tiling | 3 mm and wider | 1.6 kg/L |
| Epoxy | Wet areas, stain resistance | 2 mm and wider | 1.7 kg/L |
Starting points only, follow the tile and grout manufacturer guidance for your specific job.
Frequently asked questions
How do I know how much grout to buy?
Find the joint fraction, 1 minus (tile length × tile width) ÷ ((tile length + joint) × (tile width + joint)), then multiply by the tiled area and the joint depth to get the volume. Multiply that by the grout density (about 1.6 kg/L for sanded grout) for the weight, add roughly 10% for waste, and divide by your bag size, rounding up. This calculator does all of that and shows the number of bags.
What is the difference between sanded, unsanded and epoxy grout?
Unsanded grout is smooth and suits narrow joints under 3 mm and polished tile that sand could scratch. Sanded grout contains fine sand for strength and is used for joints 3 mm and wider, especially on floors. Epoxy grout is stain and water resistant and needs no sealing, but it sets quickly and costs several times more. The calculator uses a slightly different density for each, which you can override.
Why do small tiles need more grout than large ones?
Smaller tiles fit more rows and columns into each area, which means far more joint length to fill. An area of 50 mm mosaic has many times the joint length of the same area of 600 mm tiles, so it uses much more grout even when the joint width and depth are identical.
Does the joint depth really equal the tile thickness?
For a full-depth joint, yes, grout fills the gap from the tile face down to the adhesive bed, so the depth is effectively the tile thickness. If your tiles sit on a thick adhesive layer or you only grout the top of the joint, use the actual filled depth instead, which lowers the quantity.