How much self-leveller do I need?
Self-levelling compound is a cement- or gypsum-based powder that, mixed with water, flows out into a flat, smooth layer. It is used to correct small unevenness before laying vinyl, laminate, carpet or large-format tiles. Because it is sold by the bag and used in thin layers, the quantity is driven by the area and the average thickness.
Most compounds need about 1.5–1.7 kg of powder per square metre for each millimetre of thickness. The default here is 1.6 kg/m²/mm in 25 kg bags; check your bag, because some lightweight or fibre-reinforced products differ. If the bag states a yield in litres, tick “enter yield per bag” and the calculator converts it.
Thin layers add up quickly
A 3 mm layer over 20 m² sounds trivial but needs about 100 kg of powder — four bags. A 10 mm layer over the same area needs 320 kg, or 13 bags. On an uneven floor the thickness varies, so use the level difference mode: enter the minimum thickness at the highest point (most products need at least 2–3 mm) and the difference to the lowest point. The average and maximum thickness are calculated for you.
Self-levelling compounds have a maximum layer thickness, usually 10–30 mm depending on the product. If your floor is out by more than about 50 mm, a self-leveller alone is the wrong tool: lay a sand-cement screed first and finish with a thin levelling layer. The calculator warns you when this applies.
The self-levelling formula
m = A × t_avg × c × (1 + waste), bags = ⌈ m ÷ 25 kg ⌉
A is the area in m², t_avg the average layer thickness in mm, and c the consumption in kg/m²/mm (typically 1.6). The primer concentrate is V = A × consumption × coats ÷ (1 + dilution) × 1.1.
Worked example
- Room 4.0 × 2.5 m = 10 m², average 5 mm, 1.6 kg/m²/mm, 5% waste.
- Mass: 10 × 5 × 1.6 × 1.05 = 84 kg → 4 bags of 25 kg (16 kg left over).
- Primer: 10 m² × 0.15 L/m² × 1 coat ÷ (1 + 3) × 1.1 = 0.41 L of concentrate → one 1 L bottle.
Do not skip the primer
A primer seals the base so the water in the compound is not sucked out too quickly, which would weaken it, and it stops air rising from the substrate and leaving pinholes in the surface. The primer is added to the shopping list automatically, calculated from its consumption, number of coats and dilution with water. On very absorbent bases such as old sand-cement screed, apply two coats.
Mixing, pouring and drying
A few practical points make the difference between a glass-flat floor and a ridged one:
- Mix each bag with the exact amount of water stated, using a drill and paddle.
- Pour in strips and work wet into wet; have a helper mixing while you pour.
- Use a spiked roller to release air and help the compound level.
- Most products are walkable after about 24 hours; check the data sheet before laying the finish, especially for thicker layers.
Frequently asked questions
How many m² does a 25 kg bag of self-leveller cover?
At 1.6 kg/m²/mm, a 25 kg bag covers about 5.2 m² at 3 mm, 3.1 m² at 5 mm and 1.6 m² at 10 mm.
What is the minimum thickness for self-levelling compound?
Usually 2–3 mm. Thinner layers do not flow properly and can crack. Check the product data sheet.
Do I need primer before self-levelling compound?
Almost always. It controls absorption, improves bond and prevents pinholes. The calculator adds it automatically; you can switch it off under Extras.
Can I lay self-leveller over underfloor heating?
Many products are suitable over heating pipes with a minimum cover above them. Use the underfloor heating mode in the full screed calculator and follow the product guidance.