Screed calculator sand, cement and m³

How the calculator works it out

Volume is the area times the depth in metres, plus your wastage. For sand and cement, the calculator then turns that volume into weight at 2,000kg of dry materials per cubic metre, splits the weight by your mix ratio, and divides the cement by 25 to give bags, rounded up. For liquid screed it stops at the volume, because that is what you order.

Why 2,000kg? Flowcrete gives the density of its semi-dry sand and cement screed as 1,800 to 2,000kg per m³. The calculator uses the top of that range so you are not short on the day, and because delivered sharp sand is damp, so a tonne of it holds less than a tonne of dry sand. It splits by weight because the Mineral Products Association notes that BS 8204-1 gives screed proportions by weight.

Here is a worked example: a 4m by 5m extension floor, 65mm floating screed over insulation, a 1:4 mix and 10% wastage.

• Area: 4 × 5 = 20m².
• Volume: 20 × 0.065 = 1.30m³. Plus 10% = 1.43m³.
• Weight of dry materials: 1.43 × 2,000 = 2,860kg.
• Cement, 1 part in 5: 2,860 ÷ 5 = 572kg. 572 ÷ 25 = 22.9, so 23 bags.
• Sharp sand, 4 parts in 5: 2,860 × 4 ÷ 5 = 2,288kg, or 2.29 tonnes.
• Total weight: about 2.86 tonnes, which is worth knowing before it goes up a flight of stairs.
• Drying estimate: 80 days in good conditions (see below).

Which mix ratio to use

Use 1:4, one part cement to four parts sharp sand, unless the job says otherwise. The MPA's screed data sheet says BS 8204-1 recommends a nominal 1:4 by weight, with 1:3 as the richest and 1:4.5 as the leanest. The Concrete Centre gives the same range.

That puts 1:5 outside the British Standard range for a sand and cement levelling screed. The calculator includes it because some people are asked to price it, but if nobody has specified it, do not use it to save a few bags.

Ratios on site are usually counted in shovels or buckets, which measure volume, not weight. A shovel of cement weighs less than a shovel of damp sand, so a 1:4 by the bucket is leaner than a 1:4 by weight. Weighing a bag against a bucket once tells you where you stand.

Per cubic metre, before wastage, the calculator gives these figures.

Minimum screed thickness by type

The thickness depends on what the screed sits on. A screed bonded to a concrete slab can be thin. One laid on a membrane or on insulation has to carry its own weight and the loads on it, so it needs to be thicker. These figures come from the MPA data sheet and The Concrete Centre, both summarising BS 8204, and from the liquid screed maker's Gypsol design data sheet.

The minimum is the thinnest point, not the average. Slabs are never perfectly flat, which is why both sources tell you to specify more than the minimum.

How long screed takes to dry

For sand and cement, allow one day per millimetre for the first 50mm, then about two days per millimetre beyond that. The first part is BS 8204-1's guidance as the screed maker Ronacrete quotes it, and The Concrete Centre gives the same one day per millimetre up to 50mm. The two days per millimetre from 50mm to 75mm is, in Ronacrete's words, "generally accepted" rather than part of the standard. So the 65mm floor above is 50 + (15 × 2) = 80 days, and Ronacrete puts a 75mm screed at about 100 days.

Liquid anhydrite screed dries on a different rule. RTU's Ultraflo aftercare guide gives 1mm a day for the first 40mm and half a millimetre a day after that, at 20°C and 65% relative humidity. That is the rule the calculator uses when you pick liquid screed. A 50mm liquid screed is 40 + (10 × 2) = 60 days.

The calculator shows the same estimate. Treat it as a planning figure, not a date to book the floor fitter, because the rule has real limits.

• It assumes good drying conditions. Ronacrete warns that drying may take considerably longer when it is cold or humid, and the windows have to be open for the moisture to go anywhere.
• Past 75mm it is guesswork. Neither source gives a rate for thicker screeds. The calculator keeps adding two days per millimetre, so treat any figure above 75mm as the least you should expect, not a forecast.
• Additives change it. Flowcrete's accelerated Isocrete K-Screed is rated at one week per 25mm in good conditions. Follow the product's own data sheet if you use one.
• Test before you lay. Flowcrete cites a maximum of 75% relative humidity before sensitive floor finishes. RTU asks for under 75% RH, or under 0.5% moisture by carbide test, before tiles, wood or vinyl go on its anhydrite screed.

Liquid screed: what to order

Liquid screed comes ready mixed and is pumped in, so the only figure you need from the calculator is the cubic metres. Choose Liquid (flowing) screed, enter the area and the depth, and add a wastage allowance for an uneven base.

For the same 20m² extension laid with 50mm of anhydrite screed over underfloor heating, with 5% wastage: 20 × 0.05 = 1.00m³, plus 5% = 1.05m³ to order. Confirm the final figure with your supplier before they book the pump.

It can go down thinner than sand and cement, as the minimums above show. Gypsol advises keeping the depth as close to the minimum as possible, to avoid long drying times.

Two points catch people out. RTU says its Ultraflo screed can be force dried with the underfloor heating after 7 days, starting at 20°C and rising 5°C a day, with the windows open. And because it is a calcium sulphate screed, it needs a primer before a cement-based tile adhesive goes on, or a calcium sulphate adhesive instead.

Before you order the materials

Most screed quantity mistakes start with the depth. Take levels across the floor at several points, not one, and enter the average depth plus a margin. A slab that is 10mm out across a room can add more material than your wastage allowance covers.

Order sharp sand. The Concrete Centre describes screed as cement and sharp sand, and the MPA as cement and fine aggregate. Check the delivery note says sharp sand before it is tipped.

If you are pricing the floor for a customer, the quote template lists the materials and labour on one page, with a valid-until date so a price rise in cement does not land on you. The day rate calculator covers the labour side.

• What the calculator does not do: structural design, falls to drains, reinforcement or fibres, the damp proof membrane, insulation, bonding agents or levelling compounds.
• It does not price anything. Prices for sand, cement and liquid screed vary by area and by load size, so ask your merchant.

Common questions