What this calculator does
Grout fills the joints, which are a small fraction of the surface. Twenty square metres of 300 mm tiles with a 3 mm joint needs only 5.0 kg, because the joints occupy just 1.97% of the area.
Joint width matters more than anything else in the calculation. Going from 2 mm to 10 mm takes the requirement from 3.4 kg to 16.2 kg, nearly five times, because widening the joint increases the share of the area almost proportionally while the depth stays the same.
The formula
The share of the area taken by joints is one minus the tile area divided by the area each tile occupies including its joint. Multiplying by the surface area and the joint depth gives the grout volume, and the density converts that to weight. Joint depth is taken as the tile thickness, since grout fills the full depth.
| Term | Meaning |
|---|---|
| Joint width | The gap between tiles, which drives the quantity. |
| Joint depth | How deep the grout goes, normally the tile thickness. |
| Grout density | Around 1,600 kg/m³ for cement-based grout. Epoxy differs. |
| Sanded and unsanded | Sanded grout for joints over about 3 mm, unsanded below, since sanded grout will not pack into a narrow joint. |
The inputs explained
| Field | What to enter |
|---|---|
| Area to tile (m²) | Area to tile in square metres. |
| Tile length (mm) | Tile length in millimetres. |
| Tile width (mm) | Tile width. |
| Joint (gap) width (mm) | Joint width. 2 to 3 mm for wall tiles, 3 to 5 mm for floors, more for rustic tiles. |
| Joint depth (≈ tile thickness) (mm) | Joint depth, normally the tile thickness. |
| Grout density (kg/m³) | Grout density. 1,600 kg/m³ is typical for cement-based grout. |
When to use it
Buying grout for a tiling job
Grout is sold by the bag and the quantity is easy to overestimate badly.
Comparing tile sizes
Larger tiles have proportionally fewer joints and need less grout.
Budgeting an epoxy grout job
Epoxy costs several times cement grout, so the quantity matters more.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does joint width change the quantity?
The same tiling at a range of joint widths.
| Joint width | Grout needed | Grout joints, share of area | 1 kg bags to buy |
|---|---|---|---|
| 2 mm | 3.4 kg | 1.32% | 4 |
| 3 mm | 5.0 kg | 1.97% | 6 |
| 5 mm | 8.3 kg | 3.25% | 9 |
| 10 mm | 16.2 kg | 6.35% | 17 |
Questions
How much grout do I need per square metre?
Far less than most people assume: around 0.25 kg per square metre for 300 mm tiles with a 3 mm joint. Small tiles and wide joints raise it considerably, and mosaic sheets can need several times as much because the joint share is so much higher.
Should I use sanded or unsanded grout?
Sanded for joints wider than about 3 mm, unsanded below that. Sanded grout will not pack properly into a narrow joint, and unsanded grout shrinks and cracks in a wide one. Sanded grout can also scratch soft polished stone, which needs care.
How deep is a grout joint?
The full tile thickness, since grout fills the joint from the substrate to the surface. Entering a shallower depth underestimates the quantity. This is why thick floor tiles need more grout than thin wall tiles at the same joint width.
Does epoxy grout use the same quantity?
The volume is the same but the density differs, so the weight changes. Epoxy is also considerably more expensive and has a short working time once mixed, so it is usually mixed in small batches and an accurate quantity matters more.
For the tiles themselves, see the flooring calculator. For a paved surface instead, see the paver calculator.