Self-Levelling Compound per m²: Calculation by Area and Layer Thickness

Short answer: kg needed = area (m²) × average layer thickness (mm) × consumption (kg/m² per mm), rounded up to whole bags. The calculator uses 1.7 kg/m² per mm; self-levelling compounds typically need 1.5–1.7 kg/m² per mm, so for a specific product the data on its pack or data sheet applies. For 20 m² at 5 mm the calculator gives 170 kg, i.e. 7 bags of 25 kg.

Converting millimetres and metres

Thickness is entered in millimetres and area in m². 1 mm is 0.001 m, so 1 m² of a 1 mm layer is 0.001 m³, i.e. 1 litre. That is why consumption in kg/m² per mm equals, numerically, the kilograms of dry powder per litre of poured layer. A 20 m² floor with a 5 mm layer holds 100 litres (0.1 m³).

Multiply a thickness measured in centimetres by 10: 0.8 cm is 8 mm. The calculator offers whole millimetres from 1 to 30.

Consumption per m² and mm: product or assumption

The 1.7 kg/m² per mm in the calculator is an assumption, not the figure of a particular product. Manufacturers state their own consumption, thickness range per layer and bag size.

Source	Consumption	Layer thickness	Bag
Calculator (assumption)	1.7 kg/m² per mm	1–30 mm (choice in the calculator)	25 kg
Typical compound A	about 1.5 kg/m² per mm	up to 20 mm in one layer	25 kg
Typical compound B	1.6 kg/m² per mm	1–10 mm	23 kg

Example with packs

Illustrative calculation for a 5 × 4 m floor (20 m²) and an average thickness of 5 mm:

Calculator: 20 × 5 × 1.7 = 170 kg; 170 ÷ 25 = 6.8, so 7 bags of 25 kg (175 kg).
Typical compound A (1.5 kg/m² per mm): 20 × 5 × 1.5 = 150 kg, exactly 6 bags of 25 kg – with nothing to spare.
Typical compound B (1.6 kg/m² per mm, 23 kg bags): 20 × 5 × 1.6 = 160 kg; 160 ÷ 23 = 6.96, so 7 bags (161 kg).

Uniform thickness and an uneven subfloor

A uniform layer has the same thickness over the whole floor, e.g. when the entire floor is raised by 5 mm. On an uneven subfloor the thickness changes from point to point: thin over a high spot, thick in a dip. The quantity is then set by the average thickness, while the manufacturer specifies the minimum thickness at the highest point.

Find the average by measuring: set a target level (with a laser or a straightedge and spirit level) and measure the depth to it on an even grid of points, e.g. every 1 to 1.5 m. The denser the grid, the closer the average is to the real volume.

Why the deepest point is not the average thickness

Illustrative example: on a 5 × 4 m floor 9 points were measured on a 3 × 3 grid, with depths of 3, 4, 5, 4, 6, 8, 5, 7 and 11 mm. The deepest point is 11 mm, but only at one point. The average is 53 ÷ 9 = 5.9 mm, and 6 mm is entered in the calculator.

Entering the deepest point almost doubles the quantity. A grid average is valid when the points cover roughly equal parts of the floor; if only one area is deep, measure it separately and calculate it as a separate area with its own thickness.

Thickness used	Compound	25 kg bags
6 mm (average, rounded up)	204 kg	9
11 mm (deepest point)	374 kg	15

Common mistakes

Entering the deepest measured point as the thickness of the whole floor.
Entering the thinnest spot – the compound then runs out in the dips.
Using 1.7 kg/m² per mm for a product whose data sheet gives a different value.
Converting to 25 kg bags for a product sold in 23 or 20 kg bags.
A thickness outside the range the manufacturer allows for one layer.
Pouring without primer and edge strip when the manufacturer requires them.

Sources

These are typical values – the data on the pack or technical data sheet of the product you buy applies.

How the app calculates quantities, allowances and packs (methodology)

Frequently asked questions

How many litres of layer are in 1 m² at 1 mm?

One litre (0.001 m³). A 20 m² floor with a 5 mm layer holds 100 litres.

Which thickness should I enter if the floor is uneven?

The average thickness from measurements on a grid of points, rounded up to a whole mm – not the deepest point.

Why does the calculator give more compound than the data sheet?

It uses an assumption of 1.7 kg/m² per mm, while the products we checked state 1.5 to 1.6. For 20 m² at 5 mm that is 170 instead of 150 or 160 kg.