My first attempt at using self-levelling compound was a bathroom floor renovation - about 8m2 of uneven screed that needed bringing up before tiling. I read the bag, added what looked like the right amount of water, poured it, and watched it flow beautifully... right under the door threshold I had forgotten to seal. About a third of the compound ended up forming a perfect flat disc in the hallway.
That incident taught me two things: seal your edges before you mix anything, and understand exactly how much you need before you open the first bag. Running out mid-pour is not an option - the compound sets fast, and a join line in dried self-levelling compound is visible under tiles for ever.
Here is how to work out the quantity properly.
The formula
Total kg = floor area (m2) x pour depth (mm) x coverage rate (kg per m2 per mm)
The coverage rate is the critical variable. Most standard products are approximately 1.6 kg per m2 per mm depth. Some are lower (1.4) and some higher (1.8). Always look at the product data sheet for your specific bag before ordering.
Then add 10% wastage and round up to whole bags.
You cannot meaningfully use a partial bag. Mix the whole bag or do not open it.
Worked example
Floor: 25m2 kitchen, 4mm pour to level a slightly uneven concrete base.
- Total kg (net) = 25 x 4 x 1.6 = 160 kg
-
- 10% wastage = 176 kg
- Bags needed = 176 / 25 = 7.04 bags
- Order: 8 x 25kg bags
If the same floor needs a 6mm pour:
- Total kg = 25 x 6 x 1.6 = 240 kg
-
- 10% = 264 kg
- Bags = 264 / 25 = 10.56
- Order: 11 x 25kg bags
Quick reference - bags needed by floor size
At 1.6 kg/m2/mm coverage rate, 10% wastage, 25kg bags, rounded up to whole bags:
| Floor area | 3mm pour | 5mm pour | 8mm pour | 12mm pour |
|---|---|---|---|---|
| 5m2 | 1 bag | 2 bags | 3 bags | 4 bags |
| 10m2 | 3 bags | 4 bags | 6 bags | 9 bags |
| 15m2 | 4 bags | 6 bags | 9 bags | 13 bags |
| 20m2 | 6 bags | 8 bags | 11 bags | 17 bags |
| 25m2 | 7 bags | 9 bags | 14 bags | 21 bags |
| 30m2 | 8 bags | 11 bags | 17 bags | 25 bags |
These figures assume a flat floor with no significant hollows. A floor with multiple dips will need more - there is no substitute for levelling the worst areas with a feather-finish product first if the variation is more than 5-6mm.
What affects how much you need
Floor flatness. The calculator works on average depth. If your floor has one area that is 8mm low and most of it is flat, the average depth might be only 3-4mm but you will need considerably more compound than the average suggests. Check the floor carefully with a spirit level or straight edge first.
Pour depth. Even a 1mm increase across a large floor adds significant material. At 25m2, the difference between a 3mm and a 5mm pour is about 80 kg - more than three extra bags.
Coverage rate variation. Do not assume all products use the same rate. Feather-finish products and certain specialist compounds have different densities. Always read the bag.
Temperature. In cold conditions the compound flows less freely and may need a fraction more water - which slightly changes coverage. Do not apply below 5C.
Calculating with the self-levelling compound calculator
The quickest way is to use the Self-Levelling Compound Calculator - enter your floor area, pour depth and the coverage rate from your product and it calculates total kg and bag count with wastage instantly.
The default coverage rate is 1.6 kg/m2/mm. If your product specifies something different, change the input before reading the result.
My tips for getting the quantity right
Measure the floor worst-case, not average. Walk the floor with a 1.5m straight edge or long spirit level. Find the deepest point. If you have one area that is 15mm low, you either need a deep-fill product or you need to fill the worst hollows with a cementitious repair compound first, let it cure, then apply your leveller at a shallower average depth.
Order at least one extra bag. Running out when the bucket is half empty and the compound already in the floor is starting to set is one of the most stressful situations in a renovation. Bags that are not opened can be returned to many merchants. Open them all and you own them regardless.
Seal everything before you mix. Doorways, gaps around pipes, the joint between floor and skirting. Compound will find any gap. A strip of cardboard held with duct tape is fine for doorways. Do not rely on the product being thick enough to stop - even a fairly stiff mix will travel under a door or into a cavity.
Mix in the right size bucket. Most bags mix to a large volume. A 25kg bag needs at least a 25-litre bucket - ideally larger. A small bucket forces you to mix in two passes per bag which wastes time and makes it harder to get a consistent mix.
Work out your mixing sequence before you start. On a large floor, you need a plan for where each bucket goes and in what order so you are not walking across wet compound. Start from the far end and work back towards the door.
Use the Self-Levelling Compound Calculator to confirm your bag count before ordering.