Tile adhesive and grout coverage
How much adhesive a trowel notch puts down, how to derive grout quantity from the tile geometry instead of looking it up, and where both figures go wrong in practice.
Adhesive: the trowel decides, not the tile
Adhesive consumption is governed by the depth of the notch on the trowel you spread it with. The tile size only matters because it determines which notch you should be using. A bigger tile needs a deeper bed to accommodate its own flatness tolerance and to reach the coverage the standards require on the back of the tile.
| Notch | kg per m² | lb per 100 ft² | Equivalent side | Example tile |
|---|---|---|---|---|
| 3 mm | 1.7 | 35 | up to 50 mm | Mosaic and sheet-mounted tiles |
| 6 mm | 3.0 | 61 | up to 150 mm | 150 × 150 (6 in) |
| 8 mm | 4.0 | 82 | up to 300 mm | 300 × 300 (12 in) |
| 10 mm | 5.0 | 102 | up to 450 mm | 300 × 600, 450 × 450 |
| 12 mm | 6.0 | 123 | up to 700 mm | 600 × 600 (24 in) |
| 15 mm | 7.5 | 154 | above 700 mm | 600 × 1200 and larger, back-buttered |
Why "equivalent side" and not "longest edge"
Notch charts are normally keyed to the longest edge of the tile, and that quietly breaks on plank formats. A 300 × 600 and a 600 × 600 share a 600 mm long edge, but the first is a modest tile that beds happily on a 10 mm notch and the second is a large-format square that needs 12 mm. What actually matters is how big the tile is overall, and how far the middle of it is from an edge where trapped air can escape.
So this site keys the notch to the tile's equivalent side — the side of a square with the same area:
equivalent side = √(width × length)
A 300 × 600 gives √180,000 = 424 mm, so a 10 mm notch. A 600 × 600 gives 600 mm, so 12 mm. A 600 × 1200 gives 849 mm, which is squarely in back-buttering territory. It is one rule that behaves sensibly for squares, planks and everything between.
Three things that push consumption up
- An uneven substrate. Adhesive is not a levelling compound, but it inevitably gets used as one. A wall that is out by 5 mm over two metres will add 25–40% to the adhesive. If it is worse than that, level it properly first — thick beds of adhesive shrink, and shrinkage cracks tiles.
- Back-buttering. Large-format tiles normally need adhesive on the back of the tile as well as on the substrate to reach the required coverage. Allow roughly a third again.
- Coverage requirements. Dry interior areas typically require about 80% of the tile's back to be in contact; wet areas and exteriors require close to 100%, with no voids at the corners. Hitting 100% means either back-buttering or a deeper notch, and both cost adhesive.
Check coverage on the third tile, not the thirtieth. Lay a tile, lift it straight back off, and look at the back. You want continuous ribs squashed flat, not stripes with clean gaps between them, and nothing empty at the corners. Fix the technique now rather than discovering the problem when a tile sounds hollow six months later.
Grout: derive it, do not look it up
Manufacturers publish grout coverage as charts keyed to tile size and joint width. The charts are fine until your tile is not on them. The geometry underneath is simple enough to compute directly, and then it works for any size.
For a tile A × B with joint width W, one tile plus its share of the surrounding joint occupies a footprint of (A+W)(B+W). The tile itself is A × B of that, so the joint's share of the surface is:
f = 1 − (A × B) / ((A + W) × (B + W))
That fraction of the surface is joint, and each joint is as deep as the tile is thick. So the volume of grout per square metre is f × thickness, and multiplying by the set density of the grout gives weight:
kg/m² = thickness (mm) × f × density (g/cm³)
Use 1.6 g/cm³ for cement grout and 1.55 for epoxy. The units work out because a millimetre of depth over a square metre is a litre, and a litre at 1.6 g/cm³ is 1.6 kg.
Worked example
A 600 × 600 × 10 mm porcelain tile with a 3 mm joint:
- f = 1 − (600 × 600) / (603 × 603) = 1 − 360,000 / 363,609 = 0.00992
- kg/m² = 10 × 0.00992 × 1.6 = 0.159 kg/m²
Which is what the manufacturers' charts give for that combination. The point of deriving it is that it stays correct for a 1200 × 200 plank tile with a 2 mm joint, which no chart covers.
Grout quantity table
Generated live from the formula above, for cement grout at 1.6 g/cm³. Change the tile thickness to match yours.
| Tile size (mm) | 2 mm joint | 3 mm joint | 5 mm joint | 8 mm joint |
|---|
Add about 10% to whatever the table gives you. Grout is mixed in batches, some of every batch goes off in the bucket before it is used, and the bag sizes will force you to round up anyway.
Epoxy grout is a different material
Epoxy costs several times more than cement grout, is sold by weight in two or three parts, and has a working time measured in tens of minutes rather than hours. Its quantity per square metre is similar — the density is close to cement grout — but the waste is not: once mixed, whatever you have not used is gone. Mix small batches and expect to throw away more of it proportionally, especially on your first job with it.
It is worth the trouble in exactly the places cement grout fails: a shower floor, a commercial kitchen, a splashback behind a hob, anywhere that gets cleaned with something aggressive. Everywhere else, cement grout with a decent sealer is the sensible choice.
What neither figure includes
- Silicone. Every internal corner, and every junction between tiling and a bath, tray, worktop or basin, must be silicone rather than grout — those joints move, and grout cracks. A 310 ml cartridge covers roughly 6–8 linear metres of a typical bead.
- Primer and tanking. Substrate-dependent, and priced per square metre of substrate rather than per square metre of tile.
- Levelling compound. If you need it, you need it before the adhesive, and the adhesive figures here assume you have used it.
- Underfloor heating. You are filling around the mat or cable as well as bedding the tile. Consumption rises significantly — follow the heating system manufacturer's figure, not the adhesive bag's.
Use it
The tile calculator picks the trowel notch from your tile size, applies the substrate factor, and derives grout from the formula on this page — then converts both into whole bags.
Last reviewed 2026-09-18. Adhesive figures are typical for cement-based adhesives and vary by 10–20% between manufacturers; the bag is the authority for that product.