Screed calculator
Enter the room length and width (or the area you already know) and the thickness of the layer, and you immediately see the volume of screed and the quantity to order with a reserve. Copy the consumption off your own bag and the calculator adds the kilograms and the number of bags. The same formula covers cement screed in centimetres and self-levelling compound in millimetres — the button next to the field switches the unit and the calculator does the conversion.
How the quantity is calculated
thickness in mm = thickness in cm × 10volume = area × thickness in mm ÷ 1,000to order = volume × (1 + reserve ÷ 100)kilograms = area × thickness in mm × consumption
A screed is a layer of constant thickness across the whole floor, so its volume is ordinary geometry: area times thickness. The one trap is the unit — the thickness is typed in centimetres or millimetres while the area is in square metres, so the thickness is converted to metres first. That is why 5 cm is not 5 but 0.05 m: a factor-of-a-hundred slip is the most common mistake in a hand calculation.
If the room is not rectangular, work the area out in the area calculator first and enter it in „I know the area“ mode. In dimensions mode this page uses the same geometry engine as the rest of the Home tools, so the same room gives the same area everywhere on the site.
The reserve is added to the net volume and covers an uneven substrate, what stays in the bucket and the layer that ends up a millimetre thicker at one end of the room. It starts at 5% and stays editable — a planning convention, not a rule and not a standard. It multiplies the kilograms and the bags as well, because the material has to cover exactly the quantity you ordered.
Example: a 4.5 × 3.2 m room and a 5 cm screed
A 4.5 × 3.2 m room has 14.4 m² of floor. At 5 cm, which is 50 mm, the volume is 14.4 × 50 ÷ 1,000 = 0.72 m³. With a 5% reserve there is 0.76 m³ to order — that is the number that goes on the order, while the net volume stays visible next to it so the two are never confused.
If your bag states a consumption of 1.8 kg/m² per millimetre of thickness, the same floor needs 14.4 × 50 × 1.8 = 1,296 kg of material, or 1,360.8 kg with the reserve. In 25 kg bags that is 55 bags: 1,360.8 ÷ 25 is 54.4, and you cannot buy half a bag, so it rounds up.
A levelling compound is calculated in millimetres on the same room. A 5 mm layer at 1.5 kg/m² per mm gives 108 kg, or 113.4 kg with the reserve — six 20 kg bags. It shows at once why the two materials are not measured in the same unit: the difference between 5 mm and 5 cm is ten times the quantity.
| Thickness | Volume | To order (+5%) | kg (1.8 kg/m² per mm) | 25 kg bags |
|---|---|---|---|---|
| 3 cm | 0.43 m³ | 0.45 m³ | 816 kg | 33 |
| 4 cm | 0.58 m³ | 0.60 m³ | 1,089 kg | 44 |
| 5 cm | 0.72 m³ | 0.76 m³ | 1,361 kg | 55 |
| 6 cm | 0.86 m³ | 0.91 m³ | 1,633 kg | 66 |
| 7 cm | 1.01 m³ | 1.06 m³ | 1,905 kg | 77 |
Consumption is read off the bag, never assumed
The consumption field is empty on purpose. Kilograms per square metre per millimetre differ from product to product: cement screed, rapid-setting screed and self-levelling compound have neither the same density nor the same composition, and every bag carries its own declaration. An „average“ figure typed in here would read on screen as a fact about your material — and it would not be one.
Without it the calculator still gives a complete answer in cubic metres, because volume is pure geometry and does not depend on the manufacturer. The kilograms and the bags appear as soon as you enter the figure from the declaration, and the cost only when you add the price of a bag. We publish no material prices, because they differ by shop and by day.
The bag count is always rounded up and always calculated on the quantity with the reserve. The rounding goes through the same function as the rest of the Home cluster, which also catches the cases where a division comes out as 33.000000001 because of how decimals are stored — without that guard the calculator would sell a 34th bag for a floor that needs 33.
What this calculator does not do
It does not decide the thickness. How much screed goes over a particular substrate depends on the structure, the insulation, underfloor heating and the manufacturer's instructions — that is a decision for the designer and the contractor, not an outcome of the floor area.
It does not give a cement-to-sand ratio for a screed mixed on site. That ratio varies from job to job and there is no figure we could publish with a source, so you get the volume and order what you actually buy against it. For the same reason the amount of water is not estimated either.
It does not tell you how long a screed takes to dry. „A centimetre a week“ is a rule of thumb, not something we would publish as a fact: drying time depends on the product, the thickness, the temperature and the humidity, and it is in the manufacturer's instructions. Once the layer is done, carry the floor area on to the laminate calculator or the tile calculator.
Frequently asked questions
How many bags of screed do I need for 20 m²?
It depends on the thickness and on the consumption printed on your bag. For 20 m² at 5 cm the volume is 1 m³; if the declaration says 1.8 kg/m² per mm, that is 1,800 kg net, or 1,890 kg with a 5% reserve — 76 bags of 25 kg. Change the thickness or the consumption and the number moves proportionally.
How thick should the screed be?
The calculator does not decide that. The thickness follows from the substrate, the insulation and underfloor heating, and it is specified by the designer or the manufacturer's instructions. The calculator tells you how much material goes into the thickness you entered — and that figure scales linearly, so two options are easy to compare.
What is the difference between screed and levelling compound?
A screed is laid in centimetres and carries the levelling and the fall of the floor; a levelling compound is poured in millimetres as a final smoothing layer. The arithmetic is the same — what differs is the unit and the consumption on the bag, which is why this page has a cm/mm button instead of two separate tools.
Why does the calculator ask for the consumption instead of knowing it?
Because consumption is a fact about the product, not about your room. Cement screeds, rapid-setting screeds and levelling compounds all differ, so any assumed figure would be wrong for some readers. You get the volume without it — the kilograms and bags appear the moment you copy the value off the bag.
Is this the same as the concrete calculator?
The geometry is the same — both concrete and screed are area times thickness — but the material is bought differently. Concrete is ordered in cubic metres and paid for per m³, while screed is usually bought in bags, so this calculator works out the kilograms and the bag count from the consumption on the declaration.