Hong Life CalculatorsUnderstanding the self-levelling compound calculator — concept and principle
This calculator estimates how much self-levelling compound or flowing screed powder you need to level a floor, and how many bags to buy. Enter the floor area, the average thickness, the product's consumption rate and the bag size. It is used before laying tiles, vinyl, laminate or other floor finishes that need a flat, smooth base.
How it is calculated
- Powder (kg) = area (m²) × average thickness (mm) × consumption (kg/m²/mm).
- Bags = ⌈powder ÷ bag size⌉.
The consumption rate, printed on the bag, is the weight of dry powder needed to cover 1 m² at 1 mm thick. Most cement-based self-levellers use roughly 1.5–1.7 kg/m²/mm.
The principle
One square metre at one millimetre thick is exactly 1 litre of volume. The consumption rate is therefore the mass of dry powder needed for each litre of finished layer, so multiplying by area and thickness gives the total weight of powder. Because the compound flows to a level surface, its thickness varies across an uneven floor: thin over high spots and thick in hollows. That is why the average thickness, not the maximum, drives the quantity.
Worked example
Default inputs: area 20 m², average thickness 5 mm, consumption 1.6 kg/m²/mm, 25 kg bags.
- Powder = 20 × 5 × 1.6 = 160 kg.
- Bags = 160 ÷ 25 = 6.4, rounded up to 7 bags.
| Area (m²) | Powder (kg) |
|---|---|
| 5 | 40 |
| 10 | 80 |
| 15 | 120 |
| 20 | 160 |
| 25 | 200 |
| 30 | 240 |
| 40 | 320 |
How to read the result
To find the average thickness, measure the floor's high and low points with a laser or straightedge on a grid, or use the average of several depth readings. A rough floor may need more than you expect: a 3 mm average on 20 m² uses about 96 kg, but 8 mm needs 256 kg. Each product has a minimum and maximum thickness per layer; outside that range it can crack or fail to flow. Always add a little extra, because you must pour continuously and cannot stop halfway to fetch another bag.
Common mistakes
- Using the deepest point as the average thickness.
- Skipping the primer, which causes pinholes and poor bond.
- Mixing more water than specified, which weakens the surface.
- Not sealing gaps and edges, so the compound runs away under walls.
Frequently asked questions
How much self-leveller do I need per square metre?
About 1.6 kg per millimetre of thickness, so a 5 mm layer needs around 8 kg/m².
Can I use self-levelling compound over wooden floors?
Only products rated for timber substrates, usually with a primer and sometimes reinforcement mesh.
How long before I can walk on it?
Many products allow foot traffic after a few hours, but floor coverings often need longer; check the bag.
Sources: EN 13813 — Screed material and floor screeds: properties and requirements