Grout fills a very thin void, and the amount you need is driven by three things: how much joint there is per square metre, how wide that joint is, and how deep it goes. The first of those is set by the tile size, and it moves much more than people expect. Halving the tile side doubles the length of joint in a square metre, so 300 mm tiles need twice the grout of 600 mm tiles on the same floor — and four times as much as 1200 mm tiles. This works it out from the joint geometry rather than from a coverage figure on the bag, and compares the same floor at three tile sizes.
Tiled area, tile size and joint
Tile size moves the grout quantity further than the joint width does.
The floor or wall being grouted, not the tile you are buying.
Set by the spacer. Two to three millimetres indoors, five or more outside.
Usually the tile thickness. Rake out a few millimetres before regrouting.
Grout left on the float, in the bucket and washed off the floor.
Result
Joint length is the number nobody thinks about
In every square metre of tiling there is a certain length of joint, and that length depends only on the tile size. For square tiles of side s, the joints run in two directions and there is 2/s metres of joint per square metre. At 600 mm that is 3.33 metres. At 300 mm it is 6.67 metres — twice as much. At 1200 mm it is 1.67 metres, half as much.
That is the whole reason a bag of grout covers wildly different areas on different jobs. The coverage figure printed on the bag assumes a particular tile size and joint width, usually a small one, and it is therefore conservative for large-format work by a factor of two or three. Working from the geometry instead of the label is the difference between buying two bags and buying six.
Width and depth: which one matters
Both are linear, so doubling either doubles the volume. But in practice width is fixed by the spacers and by the look, while depth is fixed by the tile — and depth is the one that catches people out. A 20 mm stone floor is grouted to 20 mm, which is two and a half times the volume of the same area in 8 mm ceramic, and on a large job that is a genuinely different order.
Depth is also the one you can cheat. Joints do not have to be filled solid to the bottom, and on a thick tile they rarely are: a backing rod or a part-fill is normal, and the grout only needs to be deep enough to stay put. If the depth figure looks expensive, check whether the joint really is being filled full depth before accepting the number.
Mixed grout has a working life of about thirty minutes, and whatever is left at the end of it goes in the bin. Mixing one big batch is how a third of a bag is lost.
Mix half a bag at a time, keep the bucket and the float clean, and wash the floor before the haze sets rather than after.
Choosing the grout
Cementitious grout is the default: cheap, hard-wearing, available in a range of colours, and fine for most internal floors and walls. Epoxy grout costs several times as much, is much harder to work, and is worth it where the joint has to resist stains, chemicals or constant wetting — a commercial kitchen, a shower with no screen, a worktop.
Width governs which you can use. Cementitious grout is normally used from about two millimetres up to about fifteen; below about two it will not penetrate the joint properly, and above about fifteen it shrinks and cracks. Wide joints in stone or quarry get a sanded grout, which has fine aggregate in it to limit that shrinkage.
| Situation | Joint | Type | Note |
|---|---|---|---|
| Internal floor, ceramic | 2 to 3 mm | Cementitious | The default case |
| Internal floor, large format | 3 mm | Cementitious, flexible | Allow for movement |
| Wet room floor | 3 mm | Epoxy, or cementitious with admix | Waterproofing first |
| Wall tiling, porcelain | 2 mm | Cementitious, fine | Narrow joints read better |
| Stone or quarry | 5 to 12 mm | Sanded cementitious | Limits shrinkage |
| Worktop or splashback | 2 mm | Epoxy | Stain resistance is the point |
A worked example
- Joint length600 mm tiles give 3.33 m of joint per square metre, so 60 m² has 200 m of joint.
- Joint volumeAt 3 mm wide and 10 mm deep that is 6.0 litres of void.
- PowderGrout is about 1.5 kg per litre mixed, so 9.0 kg, plus ten percent gives 9.9 kg.
- BagsIn 5 kg bags that is 2 bags, or $30 at $15.
- The comparisonThe same floor in 300 mm tiles would need 18.0 kg; in 1200 mm tiles, 4.5 kg.
Nine kilos for sixty square metres is a genuinely small number, and it is why grout is rarely the line item worth optimising on a tiling job. The tile, the adhesive and the labour all dwarf it. Where it does matter is on a large job in small tiles, or in epoxy, where the same 9 kg becomes sixty dollars of resin rather than fifteen of powder.
Mixing and cleaning
Add powder to water, never water to powder, and mix to a smooth paste with no dry pockets. Let it slake for the time on the bag — usually five to ten minutes — and remix before using. Skipping the slake is the usual cause of a grout that powders at the surface and of colour that is not the colour on the bag.
Work it in with a rubber float held at a steep angle, diagonally across the joints, forcing the grout down rather than smearing it over. Fill in two passes rather than one: the first pass compacts, the second fills the void the first one left behind. Then clean off with a damp sponge, in one direction only, and change the water often — dirty water is what leaves a haze.
The tile count and the adhesive are the quantities that sit either side of this one. The tile figure gives the number of tiles and the cutting allowance, and the joint width you chose here is part of what sets it. The adhesive figure is driven by the trowel notch rather than the joint, and it is a much bigger number than the grout. If the tiling is being done as part of a wider floor, the flooring cost figure puts all of it in one place.
Do
- Work the joint length out from the tile size, not from the bag's coverage claim.
- Check whether the joint is really being filled full depth.
- Mix half a bag at a time, within the working life.
- Let it slake and remix before use.
- Seal corners and changes of plane with silicone, not grout.
Avoid
- Trusting the coverage figure on the bag, which assumes a small tile.
- Mixing one large batch and losing a third of it to the pot life.
- Grouting a movement joint that should be silicone.
- Cleaning with dirty water and leaving a haze on the tile.
- Skipping the slake, so the colour is not the colour on the bag.
Where the numbers stop being true
The joint length formula assumes tiles laid in a regular grid with continuous joints in both directions. A brick-bond or offset layout has the same total joint length, so the volume is the same; a basketweave or a Versailles pattern, made of several tile sizes in a repeating unit, has more, because the pieces do not share joint lines.
Nothing here accounts for a joint that is not full depth, for a joint raked out before regrouting — which reduces the depth by a few millimetres — or for the porosity of the tile edges. Absorbent unglazed edges take up water from the grout and increase the effective volume slightly, which is one reason the allowance exists.
And the density of 1.5 kg per litre is for a standard cementitious powder. Epoxy is denser, lightweight grouts are lighter, and ready-mixed tubs are sold by volume rather than weight. If you are using anything other than a standard powder, work in litres and convert with the figure on the container.
How the figures are produced
Joint length per square metre is 1/tile length plus 1/tile width, which for a square tile is two divided by the side. Total joint is that times the area. Joint volume is total joint times the width times the depth, converted to litres. Powder weight is volume times 1.5 kilograms per litre, with the waste percentage added. Bags are that divided by the bag size, rounded up. The bars compare the same floor at three tile sizes with everything else held constant.
Questions people ask
Why does the bag say it covers so little?
The printed coverage assumes a small tile with a wide joint. On 600 mm tiles with a 3 mm joint the real figure is several times better.
How much grout for 60 square metres?
About 9 kg in 600 mm tiles with a 3 mm joint, twice that in 300 mm tiles and half that in 1200 mm.
Do wider joints need much more?
Proportionally, yes. Doubling the joint width doubles the volume, which is why a stone floor with wide joints uses a surprising amount.
Can I fill the joint part-way?
Yes, on a thick tile. Grout only needs to be deep enough to stay put, and a backing rod is normal in a deep joint.
Cementitious or epoxy?
Cementitious for almost everything. Epoxy where the joint has to resist stains, chemicals or constant wetting.
Why did my grout crack in the corner?
Because it was grouted rather than sealed. Internal corners and changes of plane are movement joints and they want silicone.