Self-Levelling Compound Calculator
Estimate how much self-levelling compound you need to level a floor, and how many bags that is.
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Formula and method
Floor area is multiplied by the layer thickness to give a volume, and by the consumption rate to give the required mass. The mass is divided by the bag weight, raised by the waste allowance and rounded up to whole bags.
The deepest point sets the bag count
Self-levelling compound is poured to a level, so the highest point of the substrate defines the minimum thickness everywhere. A floor with a 6 mm dip needs a minimum of 6 mm plus the manufacturer's minimum cover, which is usually 3 mm — so a nominal 3 mm job is really 9 mm at the low corner and the average is what you pay for.
On a 20 m² room, 3 mm at 1.6 kg/m² per mm is 96 kg, but 9 mm is 288 kg. That is the difference between four and twelve bags, and it is decided by a level survey, not by the bag size.
Primer, water and the working time
Primer is not optional: without it the compound dries too fast at the edges, and the surface that looks fine on day one shells off on day thirty. Two coats on a porous screed are normal, and the second goes on before the first is fully dry.
Water is added to the exact ratio on the bag. Extra water buys flow and costs strength: a compound overwatered by 10 % can fail the pull-off test that the flooring warranty depends on. Mix one bag at a time and pour within the pot life, which is typically 20 to 30 minutes.
Spikes, pumps and the neighbours downstairs
Water-based compound finds every gap. Around pipes, along the wall junction and at the door threshold it will run through, so those get a bead of sealant and a foam strip before the pour. A leak into the flat below is not covered by the flooring budget.
On large areas, a pump and a second person are cheaper than twelve bags mixed by hand, because the working time does not pause while you carry buckets. Calculate the area, divide by the coverage on the bag, and decide the method before the first bag is opened.
How to measure the inputs
- Measure the floor length and width, or enter a known floor area directly.
- Find the deepest hollow with a straight edge and measure it in millimetres.
- Note the bag weight and the consumption rate from the packaging.
Worked example
Room length 13.12 ft; Room width 16.4 ft; Thickness 0.79 in; Consumption rate 1.6 kg/m² per mm; Package weight 55.12 lb; Waste 8 % → Bags required: 28
Reference data
| Thickness, mm | Compound, kg/m² | 25 kg bags per 10 m² |
|---|---|---|
| 2 | 3.2 | 2 |
| 3 | 4.8 | 2 |
| 4 | 6.4 | 3 |
| 5 | 8 | 4 |
At 1.6 kg per m² per mm. A 2 mm layer is a skim coat, not a structural levelling.
Practical tips
- Prime the subfloor first to stop the compound drying too fast.
- Mix whole bags with the exact water quantity stated by the manufacturer.
- Work in sections and use a spiked roller to release trapped air.
Sources
- EN 13813 — Screed material
Checked and updated October 22, 2026