Levelling compound takes about 1.6 kg per m² for every millimetre of thickness, so a 20 m² floor at 5 mm needs roughly 160 kg — seven 25 kg bags. Enter your area and the average layer thickness and this calculator returns your own figure in kilograms and whole bags, free in the browser. It prepares the subfloor; the flooring that goes on top is a separate job.
Calculate the quantity →Enter your floor area, the average layer thickness and — if it differs — the consumption rate printed on the bag. The result is the total weight and the number of bags, rounded up.
Enter a floor area and a layer thickness to see the quantity.
Quantity = area × thickness × consumption rate.
The consumption rate is the one number that changes the result the most, so take it from the bag rather than a rule of thumb:
Work out the kilograms and bags of levelling compound for the subfloor, from your area, average thickness and the consumption rate on the bag.
Plan or apply the flooring that goes on top. Autolay's planner and robot handle the tiles, planks or laminate and their material list and waste — not the levelling compound, and not the pouring of it. Subfloor prep stays a manual, separate step.
The quantity of levelling compound is driven almost entirely by one number you have to establish on site: the average thickness of the layer. Everything else — area, consumption rate, bag size — is either known or printed on the packaging. Getting the average wrong by a millimetre changes a 20 m² job by about 32 kg, which is more than a whole bag.
Work with a 2 m straightedge and a set of measuring wedges or a tape. Lay the straightedge across the room in several directions, including the diagonals, and note the gap underneath at a grid of points — roughly one reading per square metre is enough for a domestic room. Mark the high point, because that is the point the finished surface has to clear: the layer thickness at the high point is the product's stated minimum, and every other point needs that minimum plus its own gap.
The mistake to avoid is assuming the average is half the maximum. A floor with a single deep dip and an otherwise flat surface averages far less than one that slopes steadily from one wall to the other, even when both share the same worst-case reading. Take the mean of your grid readings — that is the number the calculator above wants.
Before the quantity matters, the product has to suit what is underneath it. Cementitious compounds are the default and tolerate almost any substrate; calcium-sulphate and gypsum-based ones flow beautifully and cost less, but they have to stay permanently dry, which rules them out of bathrooms, utility rooms and any floor exposed to standing water. Over an anhydrite screed the order reverses: the screed is sanded first to take off the sintered skin, because that thin weak layer is exactly what peels away when a bond fails.
Every product also carries a minimum and a maximum thickness, and both are limits rather than suggestions. A fine smoothing compound rated 1 to 10 mm will not fill a 25 mm dip: poured that thick it shrinks, cracks and costs three times what a deep-fill product would have. The reverse fails just as reliably — a heavily filled compound feathered out at 1 mm never closes to a smooth surface. Where the deviation falls outside the product's range, either build up in two layers with the intermediate priming the data sheet prescribes, or move to a levelling screed and treat the floor as a screed job.
Three build-ups need a product of their own. Over underfloor heating the layer becomes part of the heated construction: use a fibre-reinforced compound, keep the cover above the pipe crown that the heating manufacturer states, and leave the edge insulation strip standing until the compound has cured so the floor can move. Over old tiles the substrate has to be sound first — tap every tile, replace the hollow ones, grind the glaze and use a grip primer, because a loose tile telegraphs through any thickness of compound. Over timber boards, screw the boards down so they stop working, then use a flexible fibre-reinforced product at the higher minimum thickness it specifies, with the decoupling fleece if the sheet asks for one.
Levelling compounds are quoted in kilograms per square metre per millimetre of thickness, and the figure varies more than people expect. Most cementitious compounds sit between 1.5 and 1.7 kg, which is why 1.6 is a sensible default, but lightweight and fibre-reinforced products can drop to 1.2 and heavily filled or gypsum-based ones can reach 1.8. Across a 30 m² floor at 5 mm, the gap between 1.4 and 1.8 is 60 kg — two or three bags.
So take the rate from the bag or the data sheet for the exact product you are buying, not from a rule of thumb and not from a competitor's sheet. If you are comparing two products on price, compare them on cost per square metre per millimetre rather than on cost per bag: the cheaper bag is regularly the more expensive floor.
A safety margin on top is normal practice rather than padding. Substrate that is more absorbent than expected, a room that turns out to slope more than the grid suggested, material left in the bucket and in the pump line — five to ten percent covers all of it, and the field above lets you add it explicitly instead of quietly inflating the thickness.
Priming is the step most often skipped and most often responsible for a failed pour. On an absorbent substrate — cement screed, anhydrite, old concrete — the bare surface pulls the mixing water out of the compound within minutes. The compound stops flowing, cures short of full strength and leaves pinholes where escaping air could not close behind it. Primer regulates that absorption so the compound keeps its water for the few minutes it needs to self-level.
On a non-absorbent substrate the problem is the opposite: a tiled or sealed surface offers nothing to bond to, so the primer's job is to provide grip. These are different products, or at least different dilutions of the same product, and the data sheet says which. Dilution matters: an over-diluted primer on a thirsty screed does almost nothing, and an undiluted primer on a dense surface can leave a glossy film that the compound slides on.
Whatever the substrate, it has to be clean, sound and free of anything that breaks a bond — dust, curing agents, adhesive residues, paint, gypsum from plastering. Vacuum rather than sweep, and let the primer dry to the state the data sheet describes before mixing the first bag. None of this changes how many kilograms of compound the floor needs; all of it changes whether those kilograms stay put.
Self-levelling compound is unusually sensitive to water. A litre too much per bag and the mix separates, loses strength and dries with a weak, dusty surface; a litre too little and it will not flow and stays under the straightedge as a ridge. Measure the water rather than judging it, mix with a forced-action paddle for the full time the bag states, and let the mix stand for its maturing time if one is specified.
Working time is short — typically 15 to 30 minutes from mixing, less in a warm room. That is what makes the pour a planning problem rather than a mixing problem: every batch has to reach the previous one while the previous one is still wet, or the two do not merge and a ridge stays. Pour in strips, keep a wet edge, and pass a spike roller over the fresh surface to release trapped air.
This is also why the calculation should be generous by exactly one bag. Running out at three quarters of a room is not an inconvenience, it is a cold joint that has to be ground back and a surface that is no longer flat. An unopened bag returns to the merchant; a ground joint costs an afternoon and a hire of a grinder.
Two different times are printed on every bag and they get confused constantly. 'Walkable' is when the surface carries foot traffic without marking — usually two to four hours. 'Ready to cover' is when the residual moisture has fallen far enough that a floor covering can be laid over it without trapping water underneath. Between those two states there can be days.
A rough planning figure is a day per millimetre of thickness in normal conditions, but it is only a planning figure. Cold rooms, high humidity and poor ventilation stretch it considerably, and a vapour-tight covering — sheet vinyl, most resilient floors, a glued parquet on a sealed adhesive — is far less forgiving of a floor that is nearly dry than a click laminate over a foam underlay. Measure the residual moisture and compare it against the covering manufacturer's limit rather than counting days off a calendar.
Under a heated floor there is a further step: the heating has to be brought back up under a documented heat-up protocol before the covering goes down, and that protocol takes its own days. Build them into the schedule at the start; discovering them the week the flooring is due is how a job loses a fortnight.
Once the subfloor is level and dry, the covering becomes a separate calculation — plank size, pattern, cut list and waste — and that is exactly the job the free Autolay planner does from a photo of the plan, a BIM/IFC model or a 3D scan.
Once the subfloor is level, the free Autolay planner works out the tiles, planks or laminate for it — the material list and waste — from a photo of your floor plan, a BIM/IFC file or a 3D scan.
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