Calculate screed

What does this calculator do?

The screed calculator (called self-leveling-underlayment calculator or concrete-topping calculator in US English) gives you everything you need to know before the supply-house run: the exact dry-mix mass in kg, the bag count for 25 / 40 kg European bags or 50 / 60 lb US bags, the mixing water in litres, the estimated drying time until tile-ready and wood-ready, and — as the headline differentiator — an underfloor-heating compatibility check with the minimum coverage over the radiant pipe per DIN EN 1264-4 (which aligns with US ASHRAE / TCNA radiant-floor guidance).

Five differentiating features no other screed or self-leveling-underlayment calculator integrates this cleanly: screed-type picker with DIN EN 13813 mapping (CT, CA, MA, AS, SR, dry-screed boards) instead of a generic cement default, construction-type selector with automatic minimum-thickness recommendation per bonded / separation-layer / floating / heated combination, drying-time estimator as a mandatory output card with separate figures for tile (CM ≤ 3 %), wood (CM ≤ 1.8 %) and rapid-set mode (24 hours), underfloor-heating compat card with minimum-coverage check plus a note on the mandatory 21+14-day heat-up schedule, and a multi-room builder with its own screed type per room for realistic renovation planning. All math runs locally in your browser — no upload, no tracking, no data leaves your device.

The calculator follows three central European standards plus their US counterparts: DIN EN 13813 for screed mortars and screed materials (properties, classification), DIN 18560 for screeds in building construction (construction types, minimum thicknesses) and DIN EN 1264-4 for radiant-floor screeds (minimum pipe coverage). US-side: ASTM C109 for hydraulic-cement-mortar compressive strength, and ACI 302.1R for concrete floor and slab construction.

Which screed type for which use case?

DIN EN 13813 classifies screeds by binder and strength. The calculator knows six types because that covers the realistically-requested options across DACH, UK and US markets. Magnesia (MA) and synthetic resin (SR) are industrial-floor specific; the others are common in residential construction.

Code	Type	Layer thickness	Density	Note
CT	Cement screed (sand-and-cement / cement-based topping)	25–100 mm	2000 kg/m³	Residential default, radiant-heat compatible, bath-compatible, weather-resistant
CA	Anhydrite (calcium-sulfate / gypsum underlayment)	35–65 mm	2150 kg/m³	Self-leveling, radiant-heat yes, wet rooms only with separate waterproofing
MA	Magnesia screed	20–50 mm	2300 kg/m³	Rare, mostly industrial floors, very high compressive strength
AS	Mastic asphalt	25–40 mm	2200 kg/m³	Heat-set, walkable immediately, no mixing water, industrial + bridges
SR	Synthetic-resin screed	6–15 mm	1500 kg/m³	Thin layer, chemical-resistant, industrial floor
Dry-screed	Gypsum-fibre / wood-fibre boards	20–30 mm	800 kg/m³ fill	Renovation, no water, light (~30 kg/m²), immediately floor-finish-ready

Smart-suggest per use case: Residential standard → CT floating 50 mm. Bathroom with radiant heat → CT heated screed 61 mm (45 mm coverage + 16 mm pipe). Renovation with low build height → dry-screed system 25 mm. Garage → CT C30 bonded 80–100 mm. Commercial → CT C25–C30 bonded 60–80 mm. Outdoor terrace → CT bonded with 1.5–2 % slope, never CA (not frost-resistant). Show-surface industrial → CT C30, ground or trowel-finished, one-lot ordering mandatory (pigment drift is visible).

How is the layer thickness determined?

DIN 18560 distinguishes four construction types with different minimum thicknesses per screed type:

Construction	Min thickness CT	Min thickness CA	Note
Bonded screed	25–30 mm	35 mm	Glued directly to the substructure, highest load capacity
Screed on separation layer	35–45 mm	35 mm	PE foil / bitumen paper between substrate and screed
Floating screed	45–65 mm	35–55 mm	On impact / thermal insulation, residential default
Heated screed	45 mm + pipe Ø	50 mm + pipe Ø	On underfloor-heating pipes, minimum coverage per DIN EN 1264-4

Worked example for heated screed: 5/8 in (16 mm) PEX outer diameter, cement screed (CT) → 45 mm coverage + 16 mm pipe = 61 mm (≈ 2.4 in) total screed height. Plus impact-sound or thermal insulation underneath (typically 30–80 mm), plus the floating-membrane, plus any leveling fill — total floor build-up over the bare slab quickly hits 110–140 mm (≈ 4.5 to 5.5 in). Important to know when planning door thresholds, baseboard radiators and stair landings.

How long does the drying take?

The drying-time question is by far the most common pain point in DIY forums on both sides of the Atlantic — and no other screed or self-leveling-underlayment calculator integrates it as a mandatory output card. Background: between “walkable” (own weight) and “floor-finish-ready” (lay the finish) lie weeks to months.

Cement screed (CT) rule of thumb: one day per mm to tile-ready (CM ≤ 3 % with decoupling membrane). For wood floor (CM ≤ 1.8 %) about factor 1.3 — so 50 mm CT needs ~50 days for wood, ~40 days for vinyl, ~28 days for tile. For thicker layers (>70 mm) the drying slows disproportionately because water from the interior migrates slower.

Anhydrite (CA): faster than CT at the same conditions. 35–45 mm CA reaches tile-ready in 14 to 21 days. Above 50 mm layer thickness, drying slows significantly (each additional 10 mm adds roughly 14 days).

Rapid-set screed (CT-F4-EB with specialty binder): walkable after 6 hours, floor-finish-ready after 24 hours. Per 25-kg bag (or 50-lb US bag) roughly 1.5 to 2× more expensive than standard CT, worth it for renovations in occupied homes or projects on tight schedules.

Dry-screed system: immediately walkable, floor-finish-ready after a 24-hour acclimatisation of the board layer. No mixing water, no residual-moisture testing required.

Mastic asphalt (AS): heat-set, walkable as soon as it cools (~4 hours), immediately floor-finish-ready. No mixing water.

IMPORTANT: these are estimates under standard conditions (20 °C / 68 °F room temperature, 50 % relative humidity, normal air exchange). Before laying any floor finish ALWAYS have residual moisture verified with a CM (calcium carbide) test from the installer or floor-finish contractor — the calculator gives expected values, not test substitutes. US side: ASTM F2170 in-situ relative-humidity testing is the equivalent standard for concrete slabs.

Does screed work with underfloor heating?

For heated screed two specifics apply versus standard screed: the minimum coverage over the radiant pipe, and the heat-up schedule before any floor finish goes down.

Minimum coverage per DIN EN 1264-4 / ACI 302.1R: cement screed (CT) 45 mm, anhydrite (CA) 50 mm. Manufacturer datasheets often add a 5 mm safety margin — the calculator returns the strict DIN figure, not the manufacturer overlay. The thermal resistance of the screed layer contributes to the total R-value: the thinner the screed, the faster the heat transfer from pipe to floor — but minimum coverage is mandatory.

Heat-up schedule per DIN EN 1264-4: 21 days drying after pour, then a 14-day heat-ramp schedule (5 days at 25 °C supply temperature, 7 days at the manufacturer’s maximum supply temperature, typically 50–55 °C, 2 days cooling) before any floor finish is installed. The schedule is mandatory — incomplete or skipped, the finish manufacturer’s warranty is void, and joints commonly cup or lift. The installer documents the schedule in writing; the schedule documentation is part of the handover acceptance docs.

Anhydrite (CA) in the bathroom: technically underfloor-heating compatible, BUT not for wet rooms without a separate bonded-waterproofing layer. The calculator recommends CT (not CA) for the “bathroom with radiant heat” use case even though CA could go thinner.

How do I calculate kg, bags and mixing water?

The calculation pipeline:

1. Volume V (m³) = area (m²) × thickness (m). US: cu ft = sq ft × inches/12.
2. Dry-mix mass (kg or lbs) = V × density. Example: 25 m² × 50 mm CT = 25 × 0.05 × 2000 = 2500 kg. US: 200 sq ft × 0.5 in × 120 lb/ft²/in ≈ 12,000 lb of self-leveler.
3. Waste overage (default 5 %, adjustable 0–20 %).
4. Bag count = ceil(mass × (1 + waste/100) / bag size). Example: 2500 kg × 1.05 / 25 = 105 bags of 25 kg. US: 12,000 lb × 1.05 / 50 = 252 bags of 50 lb.
5. Mixing water in litres = bag count × bag size × water factor. Example: 105 × 25 × 0.18 = 472.5 L for CT.

Mixing-water factors per screed type: CT 0.18 L/kg, CA 0.20 L/kg, MA 0.15 L/kg, SR 0.10 L/kg, AS 0 (heat-set), dry-screed 0 (dry-set). For mixer-size planning: a 25-kg CT bag needs 4.5 L of water, a 25-kg CA bag needs 5 L. On a typical living-room calc (25 m² CT 50 mm) that’s 472.5 L of water across 105 bags — plan the hose connection and mixer capacity accordingly.

What are typical worked examples?

Example 1 — Living room 25 m² CT 50 mm floating: use case “residential standard”. Volume 1.25 m³. Mass 2500 kg dry-mix. At 25-kg bags with 5 % waste: 105 bags. Mixing water 472.5 L. Drying time for wood floor (CM ≤ 1.8 %): ~26 days. Drying time for tile: ~20 days. Walkable after ~24 hours. Rapid-set would collapse the timeline to 24 hours floor-finish-ready — consider it on tight schedules.

Example 2 — Bathroom 6 m² CT 65 mm heated screed on 16 mm radiant pipe: use case “bathroom with radiant heat”. Minimum build per DIN EN 1264-4: 45 mm coverage + 16 mm pipe = 61 mm. Selected 65 mm is safely above. Volume 0.39 m³. Mass 780 kg dry-mix. At 25-kg bags with 5 % waste: 33 bags. Mixing water 148.5 L. Drying time for tile: ~26 days, plus 21+14 days heat-up schedule before finish install. Anhydrite (CA) would be self-leveling and faster, but in the bathroom without separate bonded waterproofing it is NOT recommended.

Example 3 — Renovation 14 m² dry-screed system 25 mm: use case “renovation with low build height”. Dry-screed leveling fill underneath (height varies with substrate). Fill mass roughly 280 kg at a 25 mm layer. No mixing water. Immediately walkable after board install, floor-finish-ready after 24-hour acclimatisation. Total board weight ~30 kg/m² × 14 m² = 420 kg — acceptable for most timber-joist floors (verify structural capacity first).

US-Imperial worked example — 200 sq ft × 1/2 in self-leveling underlayment: US-standard kitchen reno pour over old vinyl. 200 sq ft × 0.5 in × ~120 lb/ft²/in ≈ 12,000 lb dry-mix. At 50-lb bags with 10 % waste (self-leveler runs into low spots): ceil(12,000 × 1.10 / 50) = 264 bags. Primer underneath, lay self-leveler in one pour at 1/2 in max for most products, walkable in 4 to 6 hours, tile-ready in 24 hours, wood-finish-ready in 16 to 72 hours per manufacturer datasheet. Verify residual moisture with ASTM F2170 in-situ RH before laying wood.

Which other construction tools are related?

For supporting tasks around a screed project: tile calculator for the floor-finish calc once the screed is finish-ready, laminate calculator for wood-floor planning with impact-sound layers, wallpaper calculator for wall coverings in the same room. Specialty calculators for insulation-layer build-up, heat-up-schedule documentation or CM-residual-moisture conversion will come in Phase B once the first weeks of search-console data confirm real demand.

Where are the frequently asked questions?

The page header includes ten FAQ entries with schema.org/FAQPage markup, sourced from Google “people also ask” patterns for screed and self-leveling-underlayment queries on both UK and US locales. Each answer responds to its question in the first ten words — voice-search-optimized for smart speakers and AI search assistants.