United Kingdom · Materials & DIY
Tile Adhesive Calculator
Change the pack size
Enter the area and the coverage rate for your trowel notch. The calculator returns kilograms and bags, and the table below gives the rate for each common notch size and tile format.
How much tile adhesive do I need per square metre?
It depends on the trowel notch, which depends on the tile size. A 6 mm notch for small wall tiles uses about 3 kg per square metre. A 10 mm notch for medium floor tiles uses around 5 kg. Large-format tiles bedded solid can use 8 kg or more per square metre.
How to use this calculator
- Measure the tiled areaWall by wall and floor separately. Deduct anything genuinely not being tiled, such as a bath panel run or a window opening.
- Pick the coverage rate from the notch you will useNotch size is set by the tile, not by preference. Bigger tiles need a deeper bed to accommodate their own variation and the substrate's.
- Add back-buttering if you are using itLarge-format and floor tiles are usually back-buttered as well as notched, which roughly doubles the adhesive. Increase the coverage rate rather than the waste allowance for that.
- Round up to whole bagsAdhesive has a working life once mixed, so a part-used bag is fine but a part-mixed bucket is waste. Ordering one spare bag is cheap insurance.
How the calculation works
Area times a coverage rate, rounded up to whole bags. The rate is the only judgement, and it is set by the trowel notch.
A notched trowel leaves ridges of adhesive whose height equals the notch depth. When the tile is pressed and slid in, those ridges collapse and spread, producing a bed roughly half the notch depth across the full area. That is why a 10 mm notch does not give a 10 mm bed — it gives about 5 mm of solid adhesive, and uses around 5 kg per square metre.
The rates assume a flat substrate. On an uneven wall the trowel has to build out the low spots, and consumption rises quickly — a wall out by 5 mm across its length can use half as much adhesive again. This is one of the strongest practical arguments for levelling a substrate before tiling rather than trying to bed out the difference.
Back-buttering changes the picture entirely. Applying a thin coat to the back of each tile as well as notching the substrate is required for large-format tiles and for anything where full bedding matters, and it roughly doubles consumption.
kg = area × coverage rate; bags = kg ÷ bag weight, rounded upCoverage by notch size
| Trowel notch | Typical tile | Coverage (kg/m²) |
|---|---|---|
| 4 mm | Mosaic and small wall tiles | ≈ 2 |
| 6 mm | Wall tiles up to 300 × 300 | ≈ 3 |
| 8 mm | Floor tiles up to 400 × 400 | ≈ 4 |
| 10 mm | Floor tiles up to 600 × 600 | ≈ 5 |
| 12 mm | Large format, plus back-buttering | ≈ 6.5 |
| Solid bed, large format | 600 × 1200 and above | 8 – 10 |
The rule of thumb behind the table is that coverage in kilograms per square metre runs at roughly half the notch depth in millimetres. It holds well on a flat substrate and understates consumption on an uneven one.
Worked example: a bathroom
A bathroom with 3 m by 2.4 m of wall to tile is 7.2 m², using 300 by 600 mm tiles and an 8 mm notch at 4 kg per square metre.
That is 28.8 kg before waste. Adding 10% gives 31.7 kg, which is two 20 kg bags with a useful amount spare.
Now add a 2 m² floor in 600 by 600 mm porcelain, notched at 10 mm and back-buttered. Back-buttering roughly doubles the rate, so call it 10 kg per square metre — 20 kg for the floor alone, or another full bag.
Total three bags for a small bathroom. The floor, at less than a third of the area, accounts for nearly as much adhesive as all the walls, which surprises most people pricing a job for the first time.
Getting the coverage right on the wall
Adhesive coverage in the sense that matters is not kilograms per square metre — it is the percentage of the tile back actually in contact with adhesive. British Standards call for a minimum of 65 per cent solid bed for internal walls and 95 per cent for floors, wet areas and external work.
Voids behind a tile are where failures start. In a wet area, water reaches a void and sits there. On a floor, a void under a tile is an unsupported area that cracks the first time something heavy lands on it. On an external wall, water in a void freezes.
The practical check is to lift a tile occasionally as you work and look at the back. If the ridges have not collapsed into a continuous bed, either the notch is too deep for the tile, the adhesive has skinned over before the tile went on, or the tile was placed without being pressed and slid.
Skinning is the most common cause and the easiest to fix: spread only as much adhesive as you can tile over in ten to fifteen minutes, less in a warm room.
Do not use ordinary cement-based adhesive over an uncoupling membrane, on a heated floor, or on plywood without checking the product is rated for it. Flexible adhesive costs slightly more and is the correct specification for anything that moves — which includes any floor with underfloor heating.
Assumptions and limitations
- Coverage rates are typical and vary between brands — the bag's own figure is more reliable
- Assumes a flat, prepared substrate; an uneven one consumes substantially more
- Back-buttering roughly doubles consumption and should be reflected in the coverage rate you enter
- Does not include primer, levelling compound, tile backer board or grout
- Takes no account of the adhesive lost to a part-mixed bucket that goes off
- Does not determine which adhesive class is correct for the substrate and tile combination
Questions people ask about this in United Kingdom
How much tile adhesive do I need per square metre?
Roughly half the trowel notch depth in kilograms — a 6 mm notch uses about 3 kg per square metre and a 10 mm notch about 5 kg. Large-format tiles bedded solid and back-buttered can use 8 to 10 kg.
An uneven substrate raises consumption significantly, sometimes by half as much again.
What size notched trowel should I use?
It follows the tile size. 4 to 6 mm for mosaics and small wall tiles, 8 mm for floor tiles up to 400 mm, 10 mm up to 600 mm, and 12 mm or a solid bed for large format. Too small a notch leaves voids; too large wastes adhesive and squeezes into the joints.
How many square metres does a 20kg bag of adhesive cover?
About 6.5 m² at a 6 mm notch, 4 m² at 10 mm, and as little as 2 m² for large-format tiles bedded solid and back-buttered. Those figures assume a flat substrate — allow more if the wall or floor is out of true.
Do I need to back-butter tiles?
For large-format tiles, floors, wet areas and external work, yes. British Standards require 95 per cent solid bed in those situations, and notching alone rarely achieves it on a tile above about 400 mm. Internal walls with small tiles need 65 per cent, which a correct notch will give.
Can I use ordinary adhesive on a heated floor?
No — use a flexible adhesive rated for it. A heated floor expands and contracts with every cycle, and a rigid adhesive cracks or debonds. The same applies over plywood, uncoupling membranes and any substrate that moves.
Why did my tiles come loose?
Usually insufficient solid bed. Either the adhesive skinned over before the tile went on, the tile was placed without being pressed and slid to collapse the ridges, or the notch was wrong for the tile size. Lifting a tile occasionally while you work is the only reliable way to catch it.
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Method, source and review date
- How it is worked out
- Coverage by trowel notch size, with a waste allowance
- Formula used
adhesive (kg) = area (m²) × coverage rate for the notch size × (1 + waste)- Basis
- Standard mix ratios and coverage rates, editable on the page
- Last reviewed
- Confidence
- Medium
Coverage varies between adhesive brands and rises sharply on uneven substrates. The manufacturer's stated coverage for the specific product is more reliable than a generic rate.
Spotted a figure that has moved, or a formula you can show is wrong? Tell us — send a correction. Fixes to rates and formulas are treated as urgent.
Disclaimer
Quantities are an estimate for planning and ordering. Site conditions, wastage, cuts and delivery minimums all change what you actually need — always confirm with your supplier or the person laying the material before ordering.