Screed type (sets a typical starting depth - edit below if you know your spec)
Area (already know the floor area? enter it as length with width set to 1)
Depth and waste allowance
| Volume | Amount |
|---|
Materials for a 4:1 sand : cement mix (sharp sand to cement, by loose volume - editable assumptions below)
| Sharp sand | 0.00 t |
| Cement (25kg bags) | 0 |
How this calculator works
For screed, the finished floor area is multiplied by the chosen depth after converting that depth from millimetres to metres. Where the floor area is already known, put it in the length field and set width to 1. A waste allowance is added on top for spillage and an uneven sub-base or insulation surface, defaulting to 10%. The materials card then splits that volume into a standard 4:1 sharp sand : cement mix by loose volume - four parts sand to one part cement - and adds a bulking allowance, because loose dry sand and cement occupy more volume than the compacted, screeded finish: this calculator defaults to 55%, meaning the loose materials needed are about 1.55 times the finished volume, within the 40 to 65% range typically quoted for sand:cement mixes. At these defaults that works out to roughly 443kg of cement per cubic metre of finished screed, in the range usually quoted for a 4:1 mix. Sand density and cement-per-bag both vary by supplier, so both are editable above if you know your own materials.
How thick should screed be?
It depends on the type. A bonded screed, laid directly onto a primed concrete or masonry sub-base with no separating layer, is thinnest at 25 to 40mm, because the bond to the base gives it strength a floating layer doesn't have on its own. An unbonded screed, laid over a damp-proof membrane or similar separating layer with no bond to what's underneath, needs at least 50mm to have enough body to stay sound. A floating screed, laid over rigid insulation with no bond below it, is thickest: 65mm is typical for a domestic floor with no pipework, rising to around 75mm where underfloor heating pipes are embedded in the screed, since it needs cover over the pipes as well as enough body to float unsupported. These are typical UK guidance figures, not a specification - a structural engineer or the underfloor heating manufacturer's own figures take precedence for anything load-bearing or pipe-related.
Screed vs concrete: what's the difference?
Screed and concrete are both sand-cement-based, but they do different jobs. Screed uses sharp sand only, no coarse aggregate, and is laid thin to give a smooth, level, walkable surface ready for a floor covering or a heating system - it isn't designed to carry structural load on its own. Concrete includes a coarse aggregate (ballast or gravel) and is laid thicker for slabs, footings and anything structural. If your job is a structural slab, footing or post hole rather than a floor finish, the concrete calculator is the right tool instead of this one.
How long does screed take to dry?
As a rule of thumb, allow one day of drying per millimetre of thickness up to about 40mm, then longer per millimetre beyond that, since moisture has further to travel out through a thicker layer and drying slows. A 50mm screed, for instance, is not simply 50 days - expect it to take meaningfully longer than the 40mm figure suggests. Ventilation, temperature and humidity all affect the real figure, so treat this as a planning guide rather than a fixed date, and check with a moisture meter before laying a moisture-sensitive floor covering on top.
What this calculator does not check
This tool gives you a volume and a materials list only. It doesn't cover liquid (flowing) screed - a different, pumped product that's calculated and supplied by the manufacturer or installer rather than mixed on site, so if that's your job, get their figures instead. It also doesn't check insulation specification or damp-proofing requirements under a floating screed, underfloor heating pipe spacing or loading, or whether reinforcement mesh is needed - all of these depend on the specific build-up and are set by the system manufacturer or building control, not by a fixed number here.
FAQ: screed calculator worked example
How much sand and cement do I need for a 4m × 3m floating screed floor? That's a 12 m2 area. At the domestic floating-screed default of 65mm, the raw volume is 12 × 0.065 = 0.78 m3. Adding the default 10% waste allowance takes that to 0.78 × 1.1 = 0.858 m3. Applying the default 55% bulking allowance gives a loose dry volume of 0.858 × 1.55 = 1.33 m3, split 4:1 into 1.06 m3 of sand and 0.266 m3 of cement. At the default 1.6 t/m3 sand density that's 1.06 × 1.6 = 1.70 tonnes of sharp sand, and at 0.0175 m3 per 25kg bag that's 0.266 ÷ 0.0175 = 15.2, rounded up to 16 bags of cement.
Related tools
Pouring a structural slab, footing or post hole instead of a floor screed? The concrete calculator works out the ballast and cement for that job.