Screed calculator
Enter the floor area and thickness to get the tonnage of ready-to-use screed — or the sand and cement to mix yourself — along with two things that decide whether the floor actually works: whether your thickness meets the BS 8204 minimum for that construction, and how long it will take to dry.
How the calculation works
Screed quantity is volume converted to weight by the material's yield. Area × thickness gives cubic metres, andTarmac's screed data sheet (Product Data Sheet 110/01) states that one tonne of screed material occupies 0.43–0.48 m³. We use 0.45 m³ per tonne — the constant their own published coverage table is built on, and the conservative end of the range, which is the right bias when the alternative is running short mid-pour.
That table is also our test fixture: the module behind this page asserts its output against all fourteen of Tarmac's printed coverage figures (10 mm through 75 mm) before any change ships. Ten of those rows match exactly; the other four are rounded by Tarmac to convenient half-numbers, and our tests hold us to within that rounding rather than pretending the difference isn't there.
Minimum thickness — the check that matters most
More screeds fail from being too thin for their construction than from any mix problem, and the minimum depends on what the screed is sitting on. From Tarmac's data sheet, applying BS 8204-1:
| Construction | Sits on | Minimum | With heating pipes |
|---|---|---|---|
| Bonded | prepared slab, bonding agent | 40 mm | — |
| Unbonded | damp-proof membrane | 50 mm | 65 mm |
| Floating | insulation / compressible layer | 65 mm | 75 mm |
The calculator applies the right row automatically and warns you if the thickness you entered falls short.
Worked example
A 6 m × 4 m room, floating screed over insulation with underfloor heating:
- Area: 6 × 4 = 24 m²
- Minimum thickness: floating with heating pipes = 75 mm
- Volume at 75 mm plus 5% waste: 24 × 0.075 × 1.05 = 1.89 m³
- Tonnage: 1.89 ÷ 0.45 = 4.2 tonnes of ready-to-use screed
- Or site-mixed 1:4: about 3.3 tonnes of sharp sand and 26 bagsof 25 kg cement
- Drying: about 4 months before a moisture-sensitive floor finish goes down — plan the programme around it
Ready-mixed or site-mixed?
Factory-produced screed arrives correctly proportioned and, because it contains a retarder, stays workable for eight to twelve hours — one delivery, no batching errors, no cement bags to carry. Site mixing is cheaper on materials and makes sense for small areas, but every batch is an opportunity to get the ratio or the water content wrong, and screed is unforgiving of both. The mix wants asemi-dry consistency: squeezed in the hand it should hold its shape, not slump or drip. Either way the material only performs if it is thoroughly compacted — poor compaction is the failure Tarmac names first.
Drying, curing and the programme
Curing and drying are different things and both catch people out. Curingmeans keeping moisture in for at least seven days under plastic sheeting so the cement hydrates properly. Drying is the months afterwards while moisture leaves before a floor finish can be laid — a month per 25 mm, and two months per 25 mm for anything above 50 mm. A 75 mm floating screed is therefore roughly a four-month wait, and no amount of heat blowers or early underfloor heating will safely shorten it. Test the moisture content before covering; the calendar is guidance, the meter is the decision.
Frequently asked questions
How much screed do I need per square metre?
At Tarmac’s published yield of 0.45 m³ per tonne, one tonne of ready-to-use screed covers about 9 m² at 50 mm, 7 m² at 65 mm or 6 m² at 75 mm. Turned around: a 24 m² room at 75 mm needs 1.8 m³, which is about 4 tonnes. Those coverage figures are straight from the manufacturer’s data sheet, and our engine reproduces their table in automated tests.
What is the minimum thickness for a floor screed?
It depends entirely on how the screed sits on the base, and it is the single most common way a screed fails. Per Tarmac’s data sheet applying BS 8204-1: bonded (straight onto a prepared slab) at least 40 mm; unbonded (over a damp-proof membrane) at least 50 mm, or 65 mm with heating pipes; floating (over insulation) at least 65 mm, or 75 mm with heating pipes. This calculator flags your figure against the right one automatically.
What mix ratio is traditional sand and cement screed?
Normally 1:4 or 1:3 cement to sharp sand by volume, mixed to a semi-dry consistency — squeezed in the hand it should hold its shape. 1:4 is the usual domestic choice; 1:3 is richer and used where higher strength is wanted. Convert those volume proportions to purchase weights and a 1:4 mix works out at roughly 14 × 25 kg bags of cement and 1.76 tonnes of sharp sand per cubic metre of finished screed.
How long does screed take to dry before I can tile or lay flooring?
Allow roughly one month per 25 mm of thickness, and for anything above 50 mm allow two months per 25 mm for the part above 50 mm — so 50 mm is about 2 months and 75 mm about 4. That is Tarmac’s guidance and it surprises people planning a build. Damp bases, high humidity and low temperatures all extend it, and the flooring contractor must test the screed’s moisture content before covering it regardless of the calendar.
Why do screeds crack or break up?
Tarmac name it directly: the most common cause of screed failure is poor compaction. After that comes insufficient thickness for the construction type, drying too fast (never use hot air blowers or switch the underfloor heating on early), and laying onto a dirty or dusty base. Screed also needs curing — covered with plastic sheeting for at least seven days to hold moisture in.
Can underfloor heating pipes sit inside the screed?
Yes, and it is common — but the minimums increase (65 mm unbonded, 75 mm floating) and conduits need at least 25 mm of cover. Worth knowing: BS 8204-1 recommends that where possible pipes and conduits should not be laid within the thickness of a levelling screed at all. Tick the heating box above and the thickness check adjusts to the right minimum.
About the numbers on this page
Yield, density, minimum thicknesses and drying times all come from Tarmac's published screed data sheet, which applies BS 8204-1; the sand and cement split uses Breedon's published cement bulk density and the same sharp sand figure as our sand calculator, all listed on thematerial density reference. The coverage table is reproduced in automated tests before any change ships — readhow CubicMath works. New to screed?What is screed? explains how it differs from concrete, why the minimum thickness depends on the construction, and why drying takes months rather than days. Your supplier's own data sheet wins for your order. Spotted a figure that looks off?Tell us.