What this calculator works out
Screed is the flat, level layer laid over a concrete slab or insulation board that a floor covering, tile adhesive or underfloor heating pipe actually sits on. Ordering it wrong is an expensive way to find out: too little and the job stalls mid-pour, too much and the leftover dry-mix or bags become dead weight in a shed. This calculator takes a floor area and a depth, then a screed type, traditional sand-cement mixed on site, ready-mixed dry bags, or a liquid screed pumped in by a supplier, and works out exactly what to order for each, with a cost estimate alongside.
Screed isn't the same thing as concrete or mortar, even though all three share cement, sand and water. Concrete is structural, carrying load in a slab, footing or post base, and contains a coarser aggregate; this site's Concrete Calculator covers pours. Mortar beds and joints brick, block and paving, covered by the Mortar Calculator. Screed's only job is to give a flat, even surface at the right level over a base that already does the structural work, which is why it's mixed and specified differently from either.
How the maths works
Floor area comes first: a room's length times its width, or a total typed straight in if it's already known or several rooms are being added together. Multiplying that area by the screed depth, once both are in the same unit of length, gives the wet volume, the actual space the finished screed fills once laid and compacted.
From there, the three screed types diverge. A traditional site-mixed screed goes through a 10% waste allowance and this site's standard 1.33 dry-to-wet bulking factor, the same convention the Mortar and Concrete Calculators use, before splitting by the chosen mix ratio into cement bags and sand tonnes. Ready-mixed bags convert the same waste-adjusted area and depth directly into a bag count using a published bagged-product yield. Liquid screed skips both steps and reports the waste-adjusted volume straight in cubic metres, since it's ordered and delivered as a wet mix rather than mixed from dry materials on site.
Working the default example through: a 4m by 3.5m room is 14 square metres, 0.91 cubic metres of wet screed at 65mm deep, 1.001 cubic metres once 10% waste is added. The full working, bulked and split by the chosen mix ratio, is shown against the results above as each figure is entered.
Bonded, unbonded and floating: choosing a depth
How thick a screed needs to be depends on how it's built up over the base, not on taste. BS 8204, the British Standard covering screeds, bases and in-situ floorings, sets out three constructions. A bonded screed is laid directly onto a prepared concrete base with a bonding agent between the two, and only needs 25 to 40mm since the base underneath does the structural work. An unbonded screed sits on a damp-proof membrane rather than being stuck down, so it needs more body on its own, a minimum of 50mm. A floating screed goes over insulation boards or underfloor heating pipework, and needs the most depth: a minimum of 65mm for a lightly loaded domestic floor, rising to 75mm for a heavier load or to fully encase heating pipes. Independent screed contractor guidance publishes the same figures, which is why this calculator uses them as its default chip values.
Underfloor heating wants the pipe fully covered with a consistent depth above and below it, both for even heat spread and so it isn't exposed to knocks during the pour; 65mm is the widely quoted minimum for a traditional floating screed over heating pipes, which is why this calculator defaults to it. Liquid screed suppliers commonly quote a thinner 40 to 50mm over the same pipework, since a liquid mix flows fully around it without the compaction a traditional screed needs.
Traditional sand-cement: mix ratio and the 1.33 bulking factor
A traditional screed is mixed semi-dry, much stiffer than mortar or concrete, so it holds its shape when compacted and trowelled rather than flowing. 1:4 cement to sand by volume is the standard ratio for most domestic floors; a stronger 1:3 suits commercial or heavy-traffic areas, a weaker 1:5 suits sheltered or lightly used spaces such as a loft floor. Loose dry sand and cement occupy noticeably more space than the finished, compacted screed they make, since water fills the gaps between the sand grains as the mix is worked. This calculator uses a 1.33 bulking factor for that step, the same figure the Mortar and Concrete Calculators use: for every cubic metre of finished screed wanted in place, buy and mix about 1.33 cubic metres of dry sand and cement.
Cement converts to 25kg bags at 1,440 kg/m³, and sand converts to tonnes at 1.6 t/m³, both figures already used by the Mortar, Concrete and Paving Calculators. Water is the one figure this calculator can only estimate loosely, since screed is mixed drier than mortar: a water/cement ratio of around 0.40, roughly 10.5 litres per 25kg bag, is a commonly quoted starting point for a workable semi-dry mix that holds together when squeezed by hand without crumbling or releasing free water, sometimes called the "squeeze test".
A worked example, with UK prices
Take a 20m² kitchen floor, 65mm deep, mixed 1:4 with waste on: 1.43m³ to mix, 22 bags of cement and 2.43 tonnes of sand (3 bulk bags). At a typical £6.50 a cement bag and £65 a sand bulk bag, that's about £340 in materials before delivery and labour. The same floor in ready-mixed bags, at a typical £9 a bag and 106 bags needed, comes to around £950, notably more than site-mixing, the trade-off for not handling loose sand and cement. A liquid screed quote for the equivalent 1.43m³, at a typical £145 per cubic metre delivered, would come in around £205, though that price usually bundles the pump and labour in a way a DIY order doesn't.
Ready-mixed bags and liquid screed
Ready-mixed, bagged screed is a pre-blended sand and cement mix sold in 25kg bags that only needs water added, useful for smaller jobs or where site mixing isn't practical. Ardex's A38 MIX floor screed, a widely stocked bagged product, publishes a material rate of about 1.85kg per square metre per millimetre of thickness, which this calculator uses directly, cross-checked against the roughly 1.8kg/m²/mm figure several independent UK screed calculators use as their own convention.
Liquid, or flow, screed is a different product entirely: a free-flowing, self-levelling cementitious or anhydrite mix, pumped directly into the room, where it self-levels without the compaction a traditional mix needs. It's ordered and quoted by the cubic metre rather than counted in bags, which is why this calculator's liquid option reports cubic metres only, alongside a rough coverage check, rather than a cement and sand breakdown. Cemfloor, a widely used brand, recommends 40 to 50mm over underfloor heating pipes, thinner than the traditional equivalent, since it flows fully around obstructions without needing the extra body a compacted mix relies on for strength.
Drying time against curing time
Curing is the early chemical reaction between cement and water that gives screed its strength, largely finished within the first few days if the surface is protected from rapid moisture loss. Drying is the much slower process of the remaining mix water evaporating out of the whole depth, until it reaches a moisture level a floor covering can safely go down onto, and that takes weeks, not days. A commonly used rule of thumb is roughly 1mm of drying per day for the first 40mm, then about half that rate beyond it, so a 65mm screed can reasonably take upwards of two months under normal indoor conditions (20°C, 60% relative humidity; colder or damper sites dry more slowly). The only reliable way to confirm a screed is ready is a proper in-situ moisture test, not the calendar. Liquid screeds are often marketed as drying faster, but manufacturer figures vary by product, so check the one being used.
Questions people ask
What's the difference between a bonded, unbonded and floating screed?
A bonded screed is stuck directly to a prepared concrete base and only needs 25 to 40mm, since the base carries the structural load. An unbonded screed sits on a damp-proof membrane instead, so it needs a minimum of 50mm on its own. A floating screed goes over insulation or underfloor heating pipework and needs the most depth, a minimum of 65mm rising to 75mm for a heavier load.
How deep should screed be over underfloor heating?
65mm is the widely quoted minimum for a traditional floating screed over underfloor heating pipes. Liquid screed suppliers commonly quote a thinner 40 to 50mm for the same job, since a liquid mix flows fully around the pipes without needing the extra body a compacted screed relies on.
What's the difference between drying and curing?
Curing is the early chemical reaction between cement and water that builds strength, largely done within the first few days. Drying is the much slower evaporation of the remaining water out of the full depth of the screed, which is what a floor covering actually needs to wait for, and typically takes weeks rather than days.
When can I tile or lay flooring on screed?
Only once it has dried through, not just cured: roughly 1mm per day for the first 40mm and about half that rate beyond it, so a 65mm screed can take two months or more. A proper in-situ moisture test before laying flooring is the reliable way to confirm it, rather than counting days on a calendar.
Should I use a traditional mix, ready-mixed bags or liquid screed?
Traditional site-mixed sand-cement is the cheapest per square metre for a confident DIYer or trade with the tools to mix and compact it. Ready-mixed bags suit smaller jobs where mixing isn't practical, at a higher material cost. Liquid screed is usually installer-fitted, self-levels for a very flat finish and can go in thinner over underfloor heating, but is quoted as a full service rather than a materials order.
Figures on this page are typical, published values, checked against BS 8204 screed thickness guidance (via the Concrete Society), independent screed contractor guides, Ardex's A38 MIX technical data for the bagged-product yield, Cemfloor's own underfloor heating guidance, and drying-time guidance from Speed Screed and Floor Screeding Contractors, not a structural specification. Real bag yields, densities and drying times vary by product and site, so confirm exact quantities with your supplier or screed installer before a large order.