Grout Calculator

Article — Grout Calculator

Grout calculator: kilograms, bags, and joint volume for any tile project

A grout calculator converts surface area, tile dimensions, joint width, and joint depth into the kilograms or pounds of cement or epoxy grout needed to fill the joints. The formula is grout volume = surface area × joint-area ratio × joint depth, where joint-area ratio depends on tile size. For 20 m² of 30 × 30 cm porcelain tile with 3 mm joints at 8 mm depth, you need 3.15 litres of grout, or about 5.4 kg of cement grout — one and a quarter 5 kg bags.

Tile installers learn the hard way that ordering grout is a gamble. Order one bag too few and the project stalls while you drive back for another. Order three too many and the unopened bags sit in a garage until they harden from humidity. The math behind the calculator removes the guesswork, but the inputs — joint width, depth, and tile dimensions — have to be accurate to the millimetre for the answer to mean anything.

The grout volume formula

Three values drive the calculation. First, the joint-area ratio: 1 minus the tile area divided by the tile-plus-joint area. For 30 × 30 cm tiles with 3 mm joints, that is 1 − (300 × 300) / (303 × 303) = 1.97%. Smaller tiles produce more joint per square metre — mosaic tiles can hit 20% or more.

Volume then equals surface area times the ratio times joint depth. Multiply by density (1.7 kg/L for standard cement grout, 1.6 kg/L for epoxy) to get weight. Round up to the next whole bag for ordering. The default waste factor of 10% covers tile that absorbed water, cleanup losses, and joints that ended up wider than spec.

Grout types: cement vs epoxy

Two families of grout cover almost every install. Cement grout is the standard — cheap, easy to apply, available in sanded and unsanded versions. Unsanded cement grout is for joints up to 3 mm wide; sanded cement (with fine silica) is for joints 3 to 12 mm wide. Density is 1.7 kg per litre. Cement grout is porous and needs sealing in wet areas.

Epoxy grout is harder, denser, waterproof out of the box, and lasts 20 years without sealing. It costs three to five times more per kilogram, and the working time once mixed is short — 45 to 60 minutes before the bucket sets up. For showers, kitchens, and commercial floors, epoxy pays back in maintenance savings. For typical residential walls, cement is the practical choice.

Joint width and tile size

Joint width sets the look and the volume of grout you buy. The standard widths are 1.6 mm (1/16 inch) for modern minimalist installs, 3.2 mm (1/8 inch) for typical residential, 5 mm for traditional terracotta, and 10 to 15 mm for large-format and outdoor tiles.

Narrow joints look modern but demand precise tile cutting. The grout itself is harder to push into a 1.5 mm gap, and any tile-size variation forces visible misalignment. Wider joints hide variation and absorb thermal expansion, which is why outdoor and large-format installs default to 5 mm or wider.

Grout joint depth

Joint depth equals tile thickness minus 1 to 2 mm for the adhesive bed. Typical depths: 6 mm for 8 mm wall tile, 8 mm for 10 mm floor tile, 10 mm for 12 mm porcelain. Under-filled joints crack at the surface; over-filled joints make cleanup harder and waste grout.

The calculator defaults to 8 mm because that matches the most common floor-tile install. Adjust upward for thicker tile (large-format porcelain at 12 mm), downward for thin wall tile (5 to 6 mm). The depth multiplies linearly with weight — doubling depth doubles the grout you need.

Grout coverage by tile size

Big tiles need less grout per square metre because they have less joint per square metre. A 60 × 60 cm tile with 3 mm joints uses about 0.15 kg/m² of grout; a 5 × 5 cm mosaic with the same joint uses 1.7 kg/m² — eleven times more. Plan for the multiplier when ordering.

• 10 × 10 cm tile, 3 mm joint, 8 mm depth = 0.9 kg/m² cement grout
• 20 × 20 cm tile, 3 mm joint, 8 mm depth = 0.5 kg/m²
• 30 × 30 cm tile, 3 mm joint, 8 mm depth = 0.3 kg/m²
• 60 × 60 cm tile, 3 mm joint, 8 mm depth = 0.15 kg/m²
• 5 × 5 cm mosaic, 2 mm joint, 6 mm depth = 1.1 kg/m²
• Subway tile (5 × 15 cm), 3 mm joint, 6 mm depth = 0.6 kg/m²

Pattern factors for diagonal and herringbone

Straight (running bond or stack bond) patterns are the calculator’s default. Diagonal layouts add 40% more joint per square metre because the joints run on the longer diagonal axis. Herringbone is +60%. Mosaic patterns more than double the joint length. Apply the pattern multiplier to the calculated base volume, not on top of the waste factor.

Common grout calculation mistakes

The biggest miss is forgetting the joint-area ratio. Tile installers sometimes multiply tile area by some empirical kg/m² figure without checking whether the figure matches their tile size. A 60 cm tile and a 10 cm tile do not use the same grout volume per square metre even at identical joint width.

The second mistake is underestimating the depth. The 8 mm default assumes you fill to the tile surface. If the joint is 12 mm deep (thick tile, deep set bed) and you calculated at 8 mm, you order 33% less grout than needed. Measure your actual joint depth before calculating, especially on hand-cut natural stone where joints vary.

Grout mixing and curing

Cement grout sets initially in 24 to 48 hours and fully cures in 7 to 14 days. Walk on tiled floors after 24 hours, but wait 72 hours before heavy use or wet exposure. Sealing comes after full cure, usually at the two-week mark, using a penetrating silane or fluoropolymer sealer. Resealing every two to three years is normal for cement grout in wet areas.

Epoxy grout sets in 4 to 8 hours and fully cures in 24. The catch is that working time is also short — once mixed, the bucket is workable for 45 to 60 minutes before it sets up. Mix small batches, work fast, clean up before set. Once epoxy hardens on tile surfaces, only a specialty solvent removes it; commercial cleanup tools cost more than the grout itself.