Screed Calculator
Last updated: 12 August 2026
Work out the materials for a traditional sand and cement floor screed: volume, tonnes of sharp sand and 25kg bags of cement, at your chosen mix ratio and thickness.
How the calculation works
Volume = area × thickness, plus 10% for consolidation and wastage. We split that volume by your mix ratio, then convert sand to tonnes at about 1.7 t/m³ and cement to 25kg bags at a bulk density around 1,440 kg/m³.
A 4:1 sharp sand to cement mix is the traditional UK floor screed. Screed is semi-dry — it should hold together when squeezed but not be sloppy. Too much water weakens it and causes curling at the edges.
Minimum screed thickness
| Screed type | Minimum thickness | Notes |
|---|---|---|
| Bonded to concrete | 25–50mm | Slab must be clean and primed |
| Unbonded (on DPM) | 65mm | Separating membrane below |
| Floating (over insulation) | 65–75mm | 75mm where underfloor heating is fitted |
| With underfloor heating | 65–75mm | Measured above the pipes |
Drying times
The rule of thumb is one day per millimetre of thickness for the first 50mm, then longer for anything deeper — so a 50mm screed needs about 50 days before impermeable floor coverings go down. Force-drying too early causes cracking. Always test moisture with a hygrometer before laying vinyl, wood or resin.
Frequently asked questions
What is the mix ratio for floor screed?
4 parts sharp sand to 1 part cement is the standard UK floor screed mix. Use 3:1 for thinner or more heavily loaded sections. The mix should be semi-dry — holding together when squeezed but not wet or sloppy.
How thick should floor screed be?
Minimum 25 to 50mm bonded to a concrete slab, 65mm unbonded over a membrane, and 65 to 75mm floating over insulation. With underfloor heating, thickness is measured above the pipes and is usually 65 to 75mm.
How much sand and cement do I need for screed?
For a 4:1 mix at 50mm thick, allow roughly 0.075 tonnes of sharp sand per square metre. A 20 square metre floor needs about 1.5 tonnes of sharp sand and 13 x 25kg bags of cement, including a 10% wastage allowance.
How long does screed take to dry?
Roughly one day per millimetre of thickness up to 50mm, then longer for deeper sections — so a 50mm screed takes about 50 days before impermeable floor coverings can be laid. Test with a hygrometer rather than guessing.
What is the difference between screed and concrete?
Screed uses sharp sand and cement with no coarse aggregate, giving a smooth levelling layer for floor finishes. Concrete contains ballast and is structural. Screed is not designed to carry structural loads on its own.
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Estimates are for guidance only. Material dimensions, densities and coverage vary by supplier — always confirm quantities before ordering, and consult a qualified professional for structural work.