The trowel is the variable, not the tile
Adhesive consumption is set by the depth of the notches on your trowel, because that is literally the volume of material left on the wall. The tile size matters only because it dictates which trowel you are allowed to use: a large tile needs a deep bed to bridge its own flatness tolerance, so it forces a bigger notch, which triples the consumption.
adhesive kg/m² ≈ notch depth mm × 1.4
That approximation holds for standard cement-based adhesive at typical density, and it is close enough for ordering. It assumes a solid bed with no voids, which is what you should be achieving anyway.
Coverage by trowel and tile size
| Tile format | Trowel notch | Adhesive per m² | 25 kg bag covers |
|---|---|---|---|
| Mosaic, up to 10 × 10 cm | 3-4 mm | 2-3 kg | 8-12 m² |
| Small wall tile, up to 20 × 20 cm | 6 mm | 3-4 kg | 6-8 m² |
| Standard wall tile, 20 × 50 cm | 6-8 mm | 4-6 kg | 4-6 m² |
| Floor tile, 30 × 30 cm | 8 mm | 5-6 kg | 4-5 m² |
| Floor tile, 30 × 60 cm | 10 mm | 6-8 kg | 3-4 m² |
| Large format, 60 × 60 cm | 10-12 mm | 8-10 kg | 2.5-3 m² |
| Extra large, 120 × 60 cm and above | 12-15 mm, back-buttered | 10-14 kg | 1.8-2.5 m² |
| Natural stone, any size | add 20% to the above | — | — |
Read the range, not the midpoint. The low end assumes a flat substrate and a tiler who works cleanly; the high end is what happens on a real wall. If you have no reason to be optimistic, use the top of the range.
Back-buttering doubles it
Anything over 60 cm on a side should be back-buttered: adhesive is combed onto the substrate and also skimmed onto the back of the tile. This is not optional on large format — it is how you get the solid bed the tile needs, and without it the corners sit on air and crack under load.
Back-buttering adds roughly 2-4 kg per square metre on top of the trowel figure. For a 120 × 60 floor that means budgeting 14 kg/m² rather than the 10 you might read off a chart, and the difference across 20 m² is three extra bags.
Solid bed coverage is a requirement, not a preference. Most manufacturers specify 80% contact for internal floors and 100% for wet areas and external work. A hollow tile in a shower is a leak with a delay built in.
What the substrate adds
- Flat, primed plasterboard or screed
- The baseline. The figures in the table apply directly with no addition.
- Slightly uneven render or old screed
- Add 20-30%. Adhesive fills the low spots and every millimetre of dip across a square metre is another 1.4 kg.
- Visibly uneven, over 5 mm deviation
- Do not fix it with adhesive. Levelling compound is cheaper per millimetre than adhesive and does the job properly; using adhesive as a filler means slow cure, shrinkage and eventual cracking.
- Highly absorbent — aerated block, old plaster, anhydrite
- Prime it. Unprimed, the substrate pulls the water out of the adhesive before it can cure and the bond never develops. Priming costs one afternoon and saves the floor.
- Tiling over existing tiles
- Add 10% and use a primer rated for non-absorbent surfaces. The old glaze gives the adhesive nothing to key into.
A worked example
A bathroom: 4.3 m² of floor in 60 × 60 porcelain, and 11 m² of walls in 30 × 60. The screed is sound but not perfectly flat.
floor: 4.3 m² × 10 kg/m² (12 mm notch) = 43 kg
back-buttering: 4.3 × 3 = 13 kg
walls: 11 m² × 7 kg/m² (10 mm notch) = 77 kg
uneven substrate, +25% = (43 + 13 + 77) × 1.25 = 166 kg
bags = 166 ÷ 25 = 6.6 → 7 bags
Seven bags for a small bathroom. Most people buy three, and the reason is that they price the adhesive against the tile area without ever looking at the trowel. At around 175 kg the adhesive also weighs more than the tiles do.
Practical notes
- Buy one bag more than the calculation. Running out mid-wall means a cold joint, a colour difference in the bed and possibly a different batch.
- Check the pot life on the bag, usually 20-30 minutes once mixed. Mix what you can lay in that time, not a full bag on a hot day.
- Never re-temper adhesive that has started to stiffen by adding water. It looks workable and has already lost most of its strength.
- Adhesive and grout are different products and neither substitutes for the other, however similar the bags look on the shelf.
- Rapid-set adhesive halves the working time. Useful when a floor has to take traffic tomorrow, punishing when you are learning.
- Store bags off the floor and dry. Cement adhesive absorbs moisture from a garage slab and a hardened bag is landfill.
Coverage figures vary between manufacturers by 20% or more, and the bag states its own. Where the bag and this table disagree, the bag is right.