Grout Calculator

How the Grout Calculator Works

Grout consumption depends on the ratio of joint area to tile area. Smaller tiles have proportionally more joint line per square metre, so they use more grout than larger tiles at the same joint width. The formula calculates this ratio by dividing the sum of one tile's length and width by the product of its length and width. This gives the linear metres of joint per square metre of tiled surface.

That ratio is then multiplied by the joint width, the joint depth, and the grout density. Standard cementitious grout has a density of approximately 1.6 kg per litre. The result is a kilogram-per-square-metre coverage rate specific to your tile size and joint dimensions.

To get the total weight, multiply the coverage rate by your tiled area and then by the waste factor. Waste covers grout left on the float, grout that dries in the bucket, and the inevitable cleanup residue. The calculator rounds up to the nearest 0.1 kg for precision, then divides by 5 to give you a bag count based on the most common retail size (5 kg bags), rounded up to a whole number.

The formula works for any rectangular tile format. For non-rectangular tiles like hexagons or arabesque shapes, the joint-to-tile ratio is higher than this formula predicts, so treat the result as a minimum estimate. If you still need to work out how many tiles you need, do that first — the tile count determines your tiled area, which feeds into the grout calculation.

How much grout that adds up to depends heavily on tile size, because smaller tiles pack more joint line into every square metre. As a guide, at the standard cementitious density this calculator uses, with the joint width and depth each row assumes:

• 50 × 50mm mosaic, 2mm joints, 4mm deep: about 0.5 kg per m²
• 100 × 100mm mosaic, 2mm joints, 6mm deep: about 0.38 kg per m²
• 150 × 150mm tiles, 2mm joints, 6mm deep: about 0.26 kg per m²
• 300 × 300mm tiles, 3mm joints, 8mm deep: about 0.26 kg per m²
• 600 × 300mm planks, 3mm joints, 8mm deep: about 0.19 kg per m²
• 600 × 600mm tiles, 3mm joints, 10mm deep: about 0.16 kg per m²

Widening the joint has a big effect: a 6mm joint roughly doubles consumption against a 3mm one, so those 300mm tiles climb to around 0.5 kg per m². A rate as high as 4 kg per m² only turns up with very small mosaic and wide, deep joints; for ordinary wall and floor tiles you are nearly always between 0.15 and 0.5 kg per m². Grout density varies by product, roughly 1.5 to 1.8 kg per litre, so treat these figures as estimates and round up.

What the Numbers Mean

Two outputs appear after calculation. Total grout weight in kilograms is the amount of dry grout powder you need to purchase, already including your chosen waste percentage. Number of 5 kg bags rounds up to whole bags — you cannot buy a partial bag from a builder's merchant, and opened bags lose moisture over time.

Different grout types have slightly different densities that affect weight. Standard cementitious grout (both sanded and unsanded) is roughly 1.6 kg per litre, which is the figure this calculator uses. Epoxy grout behaves differently: its density is product-specific rather than a single fixed value, and it is sold in pre-measured tubs rather than loose powder, so this cementitious weight does not map onto it cleanly. If you are using epoxy, follow the coverage figures on the product's own data sheet.

The bag count is conservative by design. Buying one extra bag is far cheaper than halting a grouting job mid-wall because you ran short. Unopened bags store well in a dry location for months. Once grouting is complete, you can move on to estimating paint for surrounding walls or fitting skirting to finish the room.

What to Buy

Grout comes in a narrower range of pack sizes than most tiling materials. Standard cementitious grout is sold mainly in 2.5 kg, 5 kg, and 10 kg bags, and the 5 kg bag is the size most builders' merchants stock by default, which is why this calculator reports your total in 5 kg bags. If you spot a 20 kg bag, check the label carefully: at that weight it is almost always tile adhesive or paving jointing compound rather than tile grout, which rarely comes that large.

How far a bag actually goes depends entirely on tile size, so a single coverage figure would mislead you. As a rough guide, at the standard density:

• A 5 kg bag covers roughly 17 to 18 m² of standard 300 × 300mm floor tiles with 3mm joints
• The same 5 kg bag covers only about 9 to 12 m² of small mosaic, where the joint lines are far denser
• A 10 kg bag covers roughly double either figure

Epoxy grout is a different purchase decision. It is sold in pre-measured tubs, commonly 5 kg, cures waterproof and stain-resistant without a separate sealer, and gives you a working window of around 30 to 45 minutes before it stiffens. The trade-off is cost: as of May 2026 a 5 kg tub of epoxy grout such as Mapei Kerapoxy runs about £49 to £65, against roughly £19.50 to £21.50 for a premium 5 kg cementitious grout, so epoxy costs around two to three times as much per kilogram. Reserve it for showers, wet rooms, and worktops where the waterproofing earns its keep, and use cementitious grout everywhere else. When you are pricing a whole room, our guide to choosing the right waste factor shows how tile layout drives the quantities you need.

Practical Tips for Wall Coverings

Choosing the right grout type matters more than most people realise. Sanded grout contains fine silica sand that adds bulk and structural strength. Use it for any joint 3mm wide or above — the sand fills the gap and reduces shrinkage cracking as the grout cures. Unsanded grout is a smooth paste without sand aggregate. It suits narrow joints under 3mm and will not scratch the edges of polished porcelain, glass mosaic, or metallic tiles.

Epoxy grout is a two-part system (resin plus hardener) that cures through a chemical reaction rather than by drying. It is waterproof once cured, resists staining from coffee, wine, and cleaning chemicals, and does not require a separate sealer. The trade-off is a short working time — usually around 30 to 45 minutes before the mix starts to harden. Only mix what you can apply in that window. Epoxy is the standard choice for shower enclosures, wet rooms, and commercial kitchens.

Mixing consistency is critical for all cementitious grout types. Aim for the consistency of thick peanut butter — stiff enough to hold its shape on the float but soft enough to push into joints without excessive force. Mix only what you can apply in 20 minutes. Grout that sits too long in the bucket begins to stiffen, and adding water to re-thin it weakens the final cure.

Apply grout with a rubber float held at roughly 45 degrees to the tile surface. Work diagonally across the joints rather than along them — dragging the float parallel to a joint pulls grout out instead of pushing it in. After filling a section, wipe excess grout from the tile surface with a damp sponge within 10-15 minutes. Waiting longer allows grout haze to set, making cleanup much harder.

Colour selection affects both appearance and maintenance. Darker grout hides staining from daily use but creates a visible grid pattern between lighter tiles. Lighter grout blends with pale tiles for a seamless look but shows dirt more readily and benefits from sealing. Our guide to choosing the right waste factor covers how grout colour interacts with tile layout decisions.

Factors That Change the Calculation

Mosaic tiles (100mm or smaller) consume disproportionately more grout per square metre because the joint-to-tile ratio is much higher. A 100x100mm tile has twice the joint length per square metre compared to a 300x300mm tile at the same joint width. For mosaic sheets, the joint depth is usually the thickness of the mesh backing plus the tile itself, typically 4-6mm.

Natural stone tiles with uneven, hand-cut edges need wider joints than machine-cut ceramic or porcelain. Stone joints typically run 5-10mm wide, which increases grout consumption substantially. A similar principle applies to outdoor patio slab jointing, where natural stone slabs require even wider joints than interior tiles. The irregular edges also mean joints vary in width across the same floor, so treat the calculator's output as a minimum rather than an exact figure.

If you are regrouting over existing joints rather than grouting a new installation, the depth is only the top 2-3mm being replaced, not the full original joint depth. Rake out the old grout to a consistent depth with a grout rake before applying fresh grout. Enter this reduced depth in the calculator for a more accurate estimate.

Large-format tiles (600x600mm and above) use relatively little grout per square metre. The total may seem surprisingly low — that is correct, not an error. Fewer joints per square metre means less grout, even with wider joint widths.