Floor Screed Calculator – How Much Sand and Cement Do I Need?
Traditional, bagged, liquid or self-levelling — measure the floor, set the depth, and we'll work out the materials, bags and cost.
Floor Screed Calculator
What kind of screed floor?
Laid on a membrane over the slab. Needs at least 50mm so it does not curl at the edges.
Measure rectangles length by width. Split an L-shape into two rectangles, or measure a circle straight across its widest point.
Measure your floor
15 m²
Odd-shaped garden? Split it into rectangles, triangles and circles — we add them up for you.
You need
0.83m³ of screed
15 m² at 50mm, including 10% waste — about 1.73 tonnes
- 25kg cement bags
- 14
- Sharp sand
- 1.39 t
- 850kg sand bulk bags
- 2
- Rough drying time
- 60 days
How we worked this out
Your shopping list
14 × 25kg bags of cement
1:4 mix — 347kg of cement.
£91.00
2 × 850kg bulk bags of sharp sand
1.39 tonnes of sharp sand.
£120
Screed quantities per 10m² (1:4 sand and cement, 10% waste)
| Depth | Volume | Weight | 25kg cement bags | Sharp sand |
|---|---|---|---|---|
| 25mm | 0.25 m³ | 0.58 t | 5 | 0.46 t |
| 50mm | 0.5 m³ | 1.16 t | 10 | 0.93 t |
| 65mm | 0.65 m³ | 1.5 t | 12 | 1.2 t |
| 75mm | 0.75 m³ | 1.73 t | 14 | 1.39 t |
| 100mm | 1 m³ | 2.31 t | 19 | 1.85 t |
How this calculator works
Screed is measured by volume and bought by weight, so the calculation runs in two steps. First, area × depth = cubic metres. Then we add a 10% waste allowance — floors are never perfectly flat, and you will always lay a few millimetres proud somewhere — and multiply by 2.1 tonnes per cubic metre to get the weight.
For a traditional sand and cement screed, the mix ratio splits that weight. At 1:4 the cement is one part out of five, so it is the total weight divided by five; the rest is sharp sand. We then round the cement up to whole 25kg bags and the sand up to whole 850kg bulk bags.
Bagged ready-mix screed simply divides the total weight by the bag size. Liquid screed is ordered by the cubic metre, delivered and pumped. Self-levelling compound works differently again — it is quoted as about 1.6kg per square metre per millimetre of depth, and is only intended for 2–10mm of smoothing.
Worked example: a 20m² kitchen floor at 50mm
20m² × 0.05m = 1 cubic metre. Add 10% waste and multiply by 2.1 and you have 2.31 tonnes of screed. At a 1:4 mix, the cement is 2.31 ÷ 5 = 0.462 tonnes, which is 19 × 25kg bags, and the sand is the remaining 1.85 tonnes — three 850kg bulk bags.
That comes to roughly £125 of cement and £180 of sand, so around £305 in materials. The same floor in bagged ready-mix screed would be about 93 bags at £8 — nearly £750 — but it is far easier to handle up stairs and in flats where a mixer and a sand pile are not practical.
Drying times: the bit that catches people out
Screed goes hard in a day or two but stays wet for months. The old rule of thumb is one day per millimetre for the first 40mm, then roughly twice as long per millimetre after that. A 75mm screed is therefore not 75 days but closer to 110, and in a cold, unheated room it can be longer still.
That matters because vinyl, engineered wood and resilient floor coverings will lift, bubble or cup if they are laid over a damp screed. Before those go down, a moisture test should read below 75% relative humidity. Tiles are more tolerant, but three weeks of curing is still sensible. Liquid screeds dry faster and can be force-dried with underfloor heating after about a week — follow the supplier's commissioning schedule.
Tips from the trade counter
- Use sharp sand, not building sand. Soft building sand makes a weak, shrinking screed. Screeding sand or washed sharp sand is what you want.
- Add fibres. A bag of polypropylene fibres per mix costs little and greatly reduces shrinkage cracking.
- Prime bonded screeds. An SBR slurry brushed onto a clean, damp slab stops the base sucking water out of the mix.
- Use edge strips. Floating and heated screeds need a compressible strip around the perimeter to allow movement.
- Keep it damp for the first few days. Cover with polythene so it cures slowly — quick drying at the surface is what causes crazing.
Underfloor heating and Building Regulations
New floors and significant alterations can fall under Building Regulations for insulation and, with wet underfloor heating, for the heating system itself. Insulation under a heated screed is not optional — without it you are heating the ground. Check with your local council's building control team if you are unsure whether your work is notifiable.
Frequently asked questions
How much sand and cement do I need for screed?
A cubic metre of sand and cement screed weighs about 2.1 tonnes. At a 1:4 mix that is roughly 420kg of cement (17 bags) and 1.68 tonnes of sharp sand per cubic metre, before waste.
How thick should a floor screed be?
Bonded screed can go as thin as 25mm, unbonded needs 50mm minimum, floating over insulation needs 65mm, and traditional screed over underfloor heating is usually 75mm.
What mix should I use for screed?
1:4 cement to sharp sand is the standard for domestic floors. 1:3 is stronger for heavy-duty areas, 1:5 is weaker and only suitable for thicker, lightly loaded floors.
How long does screed take to dry?
As a rough guide, one day per millimetre for the first 40mm, then slower — a 75mm screed can take four months to reach the moisture level needed for vinyl or wood. Liquid screeds dry faster with forced drying.
Can I tile straight onto new screed?
Tiles are more forgiving than vinyl or wood, but you should still let the screed cure for at least three weeks and check moisture before tiling. Use a flexible adhesive over underfloor heating.
What is self-levelling compound for?
Smoothing an existing floor, not building levels. It is designed for 2–10mm; going deeper needs a deep-fill version or a proper screed.
Do I need fibres or mesh in my screed?
Polypropylene fibres are cheap and reduce shrinkage cracking, which is why most screeders add them as standard now. Mesh is normally used in heavy-duty or thicker floating screeds.