Trowel size chart
The larger the tile, the larger the notch – so the tile is fully bedded without voids.
| Tile size | Notch | Coverage approx. |
|---|---|---|
| Mosaic, up to 10 × 10 cm (4 × 4 in) | 3–4 mm (1/8 in V or square) | 1.1–1.4 kg/m² |
| up to 20 × 20 cm (8 × 8 in, walls) | 4–6 mm (3/16–1/4 in) | 1.4–2.1 kg/m² |
| up to 30 × 60 cm (12 × 24 in) | 6–8 mm (1/4–5/16 in) | 2.1–2.8 kg/m² |
| 60 × 60 cm (24 × 24 in) | 8–10 mm (3/8 in) | 2.8–3.5 kg/m² |
| 60 × 120 cm (24 × 48 in) and up | 10–12 mm (1/2 in) + back-buttering | 4.5–5.5 kg/m² |
Values are for cement-based flexible adhesive (thinset). Lightweight adhesives cover more area – the figure on the bag can be entered in the calculator above.
Back-buttering: when and how much more?
Back-buttering means spreading adhesive on the substrate and a thin coat on the back of the tile so it's fully wetted. It's essential for large formats, outdoor floors and heavily loaded areas – and a good idea wherever voids could cause cracking.
Allow about 1 kg/m² extra. It's worth it: a hollow-sounding 24 × 48 in tile cracks easily under point loads.
Which adhesive?
- Flexible adhesive (C2 / modified thinset) – the standard for porcelain, underfloor heating and outdoors.
- Standard adhesive (C1) – for absorbent ceramic wall tiles indoors.
- Rapid-set adhesive – walkable and groutable after a few hours, shorter working time.
- Medium-bed mortar – for uneven substrates and natural stone; much higher consumption.
The instructions on the bag always take precedence.
Application tips
- Only spread as much as you can cover within the open time (usually 20–30 minutes).
- Hold the trowel at 45° and comb the ridges in one direction – parallel to the short edge on rectangular tiles.
- Slide the tile into place and press; lift one now and then to check full coverage.
- Prime highly absorbent substrates (gypsum plaster, screed) first.
Worked example: a complete bathroom
Bathroom with a 6 m² floor (60 × 60 cm) and 18 m² of walls (30 × 60 cm):
- Floor, 10 mm notch with back-buttering: 6 × (3.5 + 1.0) = 27 kg
- Walls, 8 mm notch: 18 × 2.8 = 50.4 kg
- Total 77.4 kg → four 25 kg bags (three bags at 75 kg would be too tight)
Substrate flatness
The coverage figures assume a flat substrate. A thin bed doesn't level uneven floors – the adhesive gets thicker, consumption rises and the tiles still won't be flat. Check with a long straightedge and fill larger dips beforehand.
Flatness is critical for large formats. A leveling system helps avoid lippage between tiles but doesn't replace substrate preparation.
Adhesive, grout and tiles in one calculator
TileWright calculates all of this live as you type – with a LiDAR room scan for real floor plans, doors and windows deducted automatically, a room designer and a PDF material list. Fully offline.
Download on the App StoreFrequently asked questions
How much tile adhesive per m²?
For cement-based flexible adhesive roughly notch in mm × 0.35 kg/m²: 6 mm ≈ 2.1 kg, 8 mm ≈ 2.8 kg, 10 mm ≈ 3.5 kg, 12 mm ≈ 4.2 kg.
What trowel size for 24 × 24 tile?
8–10 mm (3/8 in); for floors rather 10 mm. For 24 × 48 in and larger use 10–12 mm (1/2 in) plus back-buttering.
What trowel size for wall tile?
4–6 mm for wall tiles up to 20 × 20 cm (8 × 8 in), 6–8 mm up to 30 × 60 cm (12 × 24 in).
How many bags of adhesive for 20 m²?
With an 8 mm notch: 20 × 2.8 = 56 kg, i.e. three 25 kg bags; with back-buttering rather four.
What is back-buttering?
Spreading adhesive on both the substrate and the back of the tile for full coverage – needed for large formats and outdoors, about 1 kg/m² extra.