United Kingdom · Materials & DIY
Screed Calculator
Change the mix assumptions
Enter the floor area and screed thickness. The calculator returns sand tonnes and cement bags at your chosen mix, and the guidance below covers the minimum depths each screed type actually needs.
How much sand and cement do I need for a screed?
Multiply the floor area by the screed thickness in metres, then by about 1.3 because dry materials occupy more space than the mixed screed. At a 1:4 mix, a 20 m² floor screeded at 65 mm needs roughly 1.7 tonnes of sharp sand and around seventeen 25 kg bags of cement.
How to use this calculator
- Measure the floor areaThe full area being screeded, including under any units that sit on the finished floor. Break L-shaped rooms into rectangles.
- Set the thickness from the type of screed, not from what fitsBonded, unbonded and floating screeds all have different minimum depths, and a screed laid thinner than its minimum will crack and hollow regardless of how well it is mixed.
- Use a 1:4 mix for most workTraditional levelling screeds are commonly 1:4 or 1:4.5 cement to sharp sand. Richer than 1:3 increases shrinkage cracking rather than improving the floor.
- Note the low water figureScreed is mixed far drier than concrete — a semi-dry mix that holds together when squeezed. The default water-cement ratio here reflects that.
How the calculation works
Volume first: area by thickness. Then the same bulking step that applies to any batched mix — dry sand contains air, and the cement and water partly fill those voids, so the dry materials occupy roughly 1.3 times the finished volume.
The dry volume is split by the ratio. At 1:4 there are five parts, so cement takes one fifth and sand four fifths. Cement's bulk density of about 1,440 kg per cubic metre converts its share into bags; sand at 1,600 kg per cubic metre converts its share into tonnes.
One thing that surprises people is how heavy a screed is. A 20 m² floor at 65 mm is only 1.3 cubic metres, but that is well over two tonnes of material to get into the room, mixed, and laid within its working time. Screeding is one of the jobs where the quantity calculation most often changes the plan.
dry volume = area × thickness × 1.3; cement = dry volume ÷ 5 for a 1:4 mixMinimum screed depths
| Screed type | Typical minimum depth | Notes |
|---|---|---|
| Bonded to a prepared slab | 25 – 40 mm | Needs a clean, mechanically prepared substrate and a bonding agent |
| Unbonded, over a membrane | 50 mm minimum | Isolated from the slab by a separating layer |
| Floating, over insulation — domestic | 65 mm minimum | Insulation compressibility drives the minimum |
| Floating, over insulation — commercial | 75 mm minimum | Higher point loads |
| Over underfloor heating pipes | 65 – 75 mm above the pipe | Cover depth is measured above the pipe, not total |
These are conventional working minimums and not a substitute for a specification. The depth over underfloor heating in particular is measured from the top of the pipe, so a 16 mm pipe on 100 mm insulation needs a total screed depth well above the cover figure. Getting this wrong is the single most common cause of cracked screed over heating pipes.
Worked example: a kitchen floor
A 5 m by 4 m kitchen is 20 m². Screeded at 65 mm, that is 1.3 cubic metres finished.
Add 10% for waste and you are mixing 1.43 cubic metres. Multiply by 1.3 for the dry volume: 1.859 cubic metres. At 1:4, cement takes one fifth — 0.372 m³, which at 1,440 kg per cubic metre is 535 kg, or 22 bags at 25 kg. Sand takes four fifths — 1.487 m³, about 2.38 tonnes.
That is a substantial job: over two and a half tonnes of material through a domestic doorway, mixed and laid in a working session. Above about 15 m², a pump-delivered flowing screed starts to compete strongly on both cost and result, and it goes down considerably thinner over underfloor heating.
Traditional screed versus flowing screed
A traditional sand-cement screed is mixed semi-dry, laid to level by hand and compacted with a float. It is cheap in materials, forgiving of small mistakes, and entirely dependent on the skill of whoever levels it. It also takes a long time to dry — the rule of thumb is a day per millimetre for the first 50 mm, and longer beyond that.
A flowing screed, usually calcium sulphate based, is pumped in and self-levels. It goes down thinner, dries faster, transfers heat better over underfloor heating, and produces a flatter floor than most hand-laid screeds. It costs more per square metre, needs the room properly sealed at the perimeter, and is unforgiving about substrate preparation.
For a small area with awkward access, traditional wins. For anything over roughly 20 m², especially with underfloor heating, flowing screed usually justifies the extra cost — and this calculator does not apply to it, because the mix is supplied ready-batched rather than mixed on site.
Drying time is not curing time. A screed can be walkable in 48 hours and still be far too wet to take a floor covering. Vinyl or engineered timber laid over a screed that has not dried to the right moisture content will fail, and that failure is expensive and entirely avoidable with a moisture test.
Assumptions and limitations
- Applies to traditional sand-cement screed only, not to flowing, calcium sulphate or proprietary screeds
- The 1.3 bulking factor is typical and varies with sand grading and moisture
- Assumes cement at 1,440 kg/m³ and sharp sand at 1,600 kg/m³
- Minimum depths quoted are conventional working figures, not a design specification
- Takes no account of fibres, admixtures or bonding agents
- Depth over underfloor heating is cover above the pipe — add the pipe diameter to get the total
Questions people ask about this in United Kingdom
What is the correct mix for a floor screed?
1:4 cement to sharp sand by volume is the standard for a traditional levelling screed, with 1:4.5 also common. Richer mixes are not better — increasing the cement content raises shrinkage and makes cracking more likely rather than producing a stronger floor.
How thick should a sand and cement screed be?
It depends on the type. Bonded to a prepared slab, 25 to 40 mm. Unbonded over a membrane, 50 mm minimum. Floating over insulation in a domestic setting, 65 mm minimum. Over underfloor heating, 65 to 75 mm measured above the top of the pipe, not in total.
How long does screed take to dry?
As a working rule, one day per millimetre for the first 50 mm, and considerably longer beyond that — a 75 mm screed can take well over three months to reach the moisture content a floor covering needs. Walkable is not dry, and laying a covering too early is a common and expensive failure.
Sharp sand or building sand for screed?
Sharp sand. Its coarser, more angular grains give the screed the strength and stability it needs underfoot. Building sand is too fine and makes a weak screed that dusts and breaks up at the edges.
Why did my screed crack?
The usual causes are laying it thinner than the minimum for its type, mixing it too wet, drying it too fast in a heated room, or omitting movement joints in a large area. Over underfloor heating, insufficient cover above the pipe is the classic one — the screed is thinnest exactly where it is heated hardest.
Can I lay screed over underfloor heating pipes?
Yes, and it is the normal construction. What matters is the cover depth above the pipe — commonly 65 to 75 mm for a traditional screed — and a controlled commissioning heat-up that raises the temperature gradually once the screed has cured. Turning the heating on early to speed drying causes cracking.
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Method, source and review date
- How it is worked out
- Nominal volume batching for a traditional sand-cement screed
- Formula used
volume = area × thickness; dry volume = volume × 1.3; then split by the mix ratio- Basis
- Standard mix ratios and coverage rates, editable on the page
- Last reviewed
- Confidence
- Medium
Bulking factors and material densities vary with sand grading and moisture. Minimum screed depths depend on the substrate and the loading, and should follow the screed manufacturer's or designer's specification rather than a generic figure.
Spotted a figure that has moved, or a formula you can show is wrong? Tell us — send a correction. Fixes to rates and formulas are treated as urgent.
Disclaimer
This is a planning estimate, not a substitute for a design signed off by a qualified engineer. Anything structural, electrical or gas-related must be checked and certified by a competent person before it is built or connected.