How much screed do I need?
Screed quantity comes down to floor area times thickness. Ten square metres at 50 mm thick is half a cubic metre of laid screed. What changes from product to product is how many kilograms of dry material it takes to produce that volume — the consumption rate printed on the bag, usually given in kg per m² per mm of thickness.
Because one litre spread over one square metre is exactly 1 mm thick, the consumption rate in kg/m²/mm is also the number of kilograms of powder per litre of finished screed. If your bag gives a yield instead (for example “one 25 kg bag makes 12.5 litres”), the calculator converts it: 25 ÷ 12.5 = 2.0 kg/m²/mm.
Choose the screed type
The product determines the unit you buy in and the thickness limits you must respect:
- Sand-cement dry mix in bags: about 2.0 kg/m²/mm, 25 kg bags, from 25 mm bonded, 50 mm unbonded and 65 mm floating, up to about 100 mm.
- Fast-drying cement screed: same consumption, ready for tiling in about a week at 50 mm.
- Site mix from cement and sand: output in 25 kg cement bags and tonnes or bulk bags of sharp sand.
- Anhydrite liquid screed: delivered by the cubic metre, typically 30–35 mm minimum, with a supplier minimum order.
- Self-levelling compound: thin layers from 2 to about 30 mm, 1.6 kg/m²/mm.
The screed formula
m = A × t × c × (1 + waste), bags = ⌈ m ÷ bag mass ⌉
A is the area in m², t the average thickness in mm and c the consumption in kg/m²/mm. For volume, V = A × t ÷ 1000 × (1 + waste) in m³. For a cement and sand site mix, the dry volume is V × 1.27, split in the ratio 1 : r between cement (1440 kg/m³) and sand (1600 kg/m³).
Worked example
- Room 5.0 × 4.0 m = 20 m², uniform 50 mm sand-cement dry mix, 2.0 kg/m²/mm, 5% waste.
- Volume: 20 × 50 ÷ 1000 × 1.05 = 1.050 m³.
- Mass: 20 × 50 × 2.0 × 1.05 = 2,100 kg → 84 bags of 25 kg.
- As a 1:4 site mix for 1.000 m³: dry volume 1.270 m³ → cement 365.8 kg (15 bags) and sand 1,625.6 kg (1.7 t, or 2 bulk bags of 0.85 t).
Bonded, unbonded or floating?
A bonded screed is stuck directly to a clean, primed concrete base and can be thinner. An unbonded screed is laid on a separating membrane and needs to be thicker to resist cracking. A floating screed sits on insulation boards — common with underfloor heating — and needs the greatest thickness plus a compressible edge strip around the walls. The calculator adds the PE membrane and edge strip automatically when the construction needs them, and a bonding primer for bonded screed.
Weight and drying time
A 50 mm sand-cement screed weighs roughly 100 kg per square metre, which matters on timber floors and upper storeys. The results show the total wet weight and the load per m². They also give a rule-of-thumb drying time — about one day per millimetre up to 40 mm and two days per millimetre beyond — so you can plan when tiles or flooring can go down.
Frequently asked questions
How many bags of screed per square metre?
At 50 mm, one square metre needs about 100 kg of sand-cement screed, which is four 25 kg bags. At 25 mm it is two bags.
How much waste should I add for screed?
Around 5% covers spillage, mixing losses and minor variations in the base. Increase it if the subfloor is very uneven and you have not measured it.
What is the minimum screed thickness?
For sand-cement screed, typically 25 mm bonded, 50 mm unbonded and 65 mm floating. Liquid anhydrite screeds can go thinner. Always follow the product data sheet.
What if my floor is not level?
Switch the thickness mode to level difference or height grid, or use the uneven floor screed calculator. A flat average underestimates the screed on a sloping floor.
Can I calculate liquid screed for underfloor heating?
Yes. Choose the underfloor heating mode and enter the pipe diameter and the cover above the pipes; you will be warned if the cover is below the usual minimum.