The formula
Grout fills the space between tiles, so the quantity is the volume of that space multiplied by the density of the mixed product. The standard industry formula:
kg/m² = ((A + B) ÷ (A × B)) × C × D × 1.6
- A
- Tile length in mm
- B
- Tile width in mm
- C
- Tile thickness in mm — the joint is as deep as the tile
- D
- Joint width in mm
- 1.6
- Approximate density of mixed cement grout in kg per litre
The important term is the first one. Because it divides the perimeter by the area, small tiles have far more joint per square metre than large ones — a 10 × 10 mosaic has ten times the joint length of a 60 × 60 tile over the same floor.
Grout per square metre, ready reckoned
| Tile size | Thickness | 2 mm joint | 3 mm joint | 5 mm joint |
|---|---|---|---|---|
| 10 × 10 cm | 6 mm | 0.4 kg | 0.6 kg | 0.9 kg |
| 15 × 15 cm | 7 mm | 0.3 kg | 0.5 kg | 0.7 kg |
| 20 × 20 cm | 7 mm | 0.2 kg | 0.3 kg | 0.6 kg |
| 30 × 30 cm | 8 mm | 0.2 kg | 0.3 kg | 0.4 kg |
| 20 × 50 cm | 9 mm | 0.2 kg | 0.3 kg | 0.5 kg |
| 30 × 60 cm | 9 mm | 0.1 kg | 0.2 kg | 0.4 kg |
| 60 × 60 cm | 10 mm | 0.1 kg | 0.2 kg | 0.3 kg |
| 120 × 60 cm | 11 mm | 0.1 kg | 0.15 kg | 0.25 kg |
| 2 × 2 cm mosaic | 4 mm | 1.3 kg | 1.9 kg | 3.2 kg |
Read across a row and the joint width roughly doubles the quantity from 2 mm to 5 mm. Read down a column and the mosaic needs ten times what a large format needs. Both effects are real and both are routinely ignored at the till.
Choosing the joint width
The joint is not decoration. It absorbs the dimensional tolerance of the tile and the movement of the substrate, which is why the minimum is set by how the tile was made rather than by taste.
| Tile type | Minimum joint | Note |
|---|---|---|
| Rectified porcelain | 2 mm | Cut to size after firing, so tolerance is tiny |
| Pressed ceramic | 3 mm | Fired to size, dimensions vary between pieces |
| Natural stone | 3-5 mm | Handmade tolerance, often uneven edges |
| Handmade / rustic terracotta | 5-10 mm | Variation is the point of the material |
| Any floor over underfloor heating | 3 mm minimum | Thermal movement needs somewhere to go |
| External paving | 5 mm minimum | Freeze-thaw and larger temperature swings |
Going below the manufacturer's minimum joint voids the warranty and, more practically, means one slightly oversized tile in the box forces the whole run off line with nowhere to recover.
Cement or epoxy
- Cement grout, unsanded
- For joints up to 3 mm. Cheapest, easiest to work, easiest to stain. The default for wall tiling.
- Cement grout, sanded
- For joints of 3 mm and over. The sand resists shrinkage in a wider joint. Do not use on polished or soft stone — it scratches.
- Epoxy grout
- Waterproof, stain-proof, chemical-resistant and roughly four times the price. Density is higher, so add 15% to any quantity calculated from the table. Working time is short and mistakes set permanently.
- Flexible / rapid-set additives
- Same quantity, different behaviour. Required over underfloor heating and on any substrate that moves, such as timber floors.
A worked example
A kitchen floor, 12.6 m² in 60 × 60 porcelain, 10 mm thick, with 3 mm joints. Plus a splashback, 3 m² of 7.5 × 15 metro tile, 8 mm thick, 2 mm joints.
floor: ((600 + 600) ÷ (600 × 600)) × 10 × 3 × 1.6 = 0.16 kg/m²
12.6 × 0.16 = 2.0 kg
splashback: ((75 + 150) ÷ (75 × 150)) × 8 × 2 × 1.6 = 0.51 kg/m²
3 × 0.51 = 1.5 kg
Two kilos for a whole kitchen floor and one and a half for three square metres of splashback. The small tiles consume three times as much per square metre. A single 5 kg bag covers both, which is why grout is rarely the thing that stops a job — but two different colours means two bags, and part-used bags of cement grout do not survive to the next project.
Practical notes
- Add 10% to the calculated figure. The formula assumes a perfectly filled joint and no waste on the float, board or floor.
- Buy the whole job in one bag where possible. Cement grout colour varies between batches and the difference shows most on a wall lit from the side.
- Mix the full bag at once if the area is small enough to grout in one session. Two mixes from the same bag can still differ in shade if the water ratio drifts.
- Colour changes the perceived quality of the tiling more than the tile does. A joint darker than the tile draws the grid; a matched joint makes the surface read as one plane.
- Seal cement grout in wet areas and kitchens. Unsealed light grout in a shower is grey within a year.
- Movement joints are not grout. Perimeters, internal corners and any run over 8 m need flexible sealant instead, and grouting them solid is what makes tiles tent later.