How do I calculate how much screed I need?
Multiply the floor area in m² by the depth in metres, then add waste. For a room that is not a rectangle, split it into rectangles and add them up.
A worked example. A 5m by 4m room is 20m². At 75mm deep, 20 × 0.075 = 1.5m³. With 10% waste that is 1.65m³.
How much sand and cement for 1m³ of screed?
For a 1:4 mix, about 380kg of cement and 1.7 tonnes of sharp sand per cubic metre. A stronger 1:3 mix uses more cement and a little less sand.
| Mix | Cement | Sharp sand |
|---|---|---|
| 1:3 | 480kg (20 bags) | 1.6 tonnes |
| 1:4 | 380kg (16 bags) | 1.7 tonnes |
| 1:5 | 320kg (13 bags) | 1.78 tonnes |
Use sharp sand, not soft building sand. Screed is mixed semi-dry, so it holds together when you squeeze it, and the dry materials bulk up by about a third on the way to a laid cubic metre.
How thick should floor screed be?
It depends on what is underneath. The minimums for sand and cement come from BS 8204-1; liquid screed minimums come from the supplier's guidance, and vary by product.
| Laid | Sand and cement | Liquid screed |
|---|---|---|
| Bonded to the slab | 25mm (40mm is the usual specification) | 25mm |
| Unbonded, over a membrane | 50mm | 30mm |
| Floating, over insulation | 65mm domestic, 75mm commercial | 35mm domestic, 40mm commercial |
| Over underfloor heating | At least 25mm over the pipes, 65mm or more overall | At least 30mm over the pipes |
The calculator warns you when a depth is under the minimum for the type you chose, and still works the numbers out.
How many tonnes of screed per m³?
About 2 tonnes for sand and cement screed, and about 2.1 tonnes for liquid screed. The 1.65m³ room above is about 3.3 tonnes of laid screed.
Sand and cement or liquid screed?
Liquid screed is pumped in and levels itself, so a whole floor goes down in hours and can be thinner. It suits underfloor heating well because it wraps round the pipes. It needs a supplier and a pump, and it is priced by the m².
Sand and cement screed can be mixed on site in any quantity, suits small rooms and patches, and needs no specialist delivery. It is slower to lay, thicker, and slower to dry.
How much does a 20kg bag of self-levelling compound cover?
About 4.2m² at 3mm deep, 6.3m² at 2mm and 2.5m² at 5mm, using 1.6kg per m² per mm. Products range from about 1.4 to 1.8kg, so check the bag. Prime the floor first: about 1 litre per 5m² neat.
How much does screeding cost per m²?
Laying sand and cement screed costs about £10 to £12 per m² in labour, on top of the materials. Liquid screed, supplied and pumped at about 50mm, runs about £16 to £25 per m². These are 2026 estimates excluding VAT; small floors often cost more per m².
How long does screed take to dry?
The rule of thumb is 1 day per mm up to 40mm, then 2 days for every mm beyond that. So 75mm takes about 110 days before a moisture-sensitive floor such as wood or vinyl goes down.
Warmth and air movement help; cold and damp slow it right down. A moisture test decides when the floor can go down, not the calendar. Fast-drying screeds are much quicker: follow the supplier's figures.
Frequently asked questions
How much screed do I need?
Floor area in m² times the depth in metres, plus about 10% waste. A 5m × 4m room at 75mm is 1.5m³, or 1.65m³ with waste.
How much sand and cement for 1m³ screed?
For a 1:4 mix, about 380kg of cement and 1.7 tonnes of sharp sand for each cubic metre of laid screed.
How do I calculate how much screed I need?
Floor area in m² times depth in metres, plus 10% for waste. A 20m² room at 75mm is 1.5m³, or 1.65m³ with waste. The calculator then turns that into bags of cement and bulk bags of sand.
How many tonnes of screed per m³?
About 2 tonnes of laid sand and cement screed, and about 2.1 tonnes of liquid screed. The dry sand you order weighs less than that per m³ of floor, because the mix compacts.
How many square metres does a 20kg self-levelling screed cover?
About 4.2m² at 3mm, or 6.3m² at 2mm, at the usual 1.6kg per m² per mm. Deeper pours cover less. Check the rate printed on the bag, as products differ.
How much screeding per square metre?
At 75mm, one m² of floor takes 0.075m³ of screed. At 1:4 that is about 29kg of cement and 128kg of sharp sand per m², before waste.
How much to lay screed per m2?
Labour to lay sand and cement screed runs about £10 to £12 per m², plus materials. Liquid screed, supplied and pumped, is about £16 to £25 per m². These are 2026 estimates excluding VAT.
How to calculate screeding per square metre?
Multiply one square metre by the depth in metres: at 50mm that is 0.05m³ per m². Multiply by the cement and sand per m³ for your mix to get the materials for each square metre.
Do I need fibres in screed?
They are worth adding. Polypropylene fibres help control shrinkage cracking while the screed dries, and many floating and underfloor heating specifications ask for them. Use the dose printed on the pack.
Can I lay screed straight onto insulation?
Yes, as a floating screed, with a separating layer such as polythene over the insulation so the screed does not run into the joints. Sand and cement needs at least 65mm for a domestic floor.
What sand do I use for screed?
Sharp sand, also called grit sand. Soft building sand is too fine: it makes a weak, dusty screed that shrinks more.
How much water in a screed mix?
Very little. Sand and cement screed is laid semi-dry: it should hold its shape when squeezed in your hand without water running out. Too wet and it shrinks and cracks as it dries.
Sources
- Build Journal: UK concrete costs 2026 (opens in a new tab)
- Concretemath: BS 8500 designated mixes by use (opens in a new tab)
- Concrete Society: external paving and BS 8500 (opens in a new tab)
- Concrete Society: screeds (opens in a new tab)
- Speed Screed: liquid screed thickness (opens in a new tab)
- Speed Screed: screed drying times (opens in a new tab)
- Blue Circle Postcrete 20kg (coverage on the product page) (opens in a new tab)
- Flowcrete: Isocrete PP Fibres data sheet (75g per 25kg of cement) (opens in a new tab)
- Mapei Ultraplan coverage, via floor prep calculators (opens in a new tab)
Quantities are estimates for ordering. The floor specification, the screed supplier and the flooring manufacturer have the final say on depth, mix and drying.
By Janusz Wozniak, Founder & Lead Developer
Built so a floor with several rooms can be ordered in one go, with the minimum depths and the drying time in front of you.
Last updated .
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