Concrete Slab Calculator

The UK works in cubic metres directly — length × width × thickness in metres gives you the order figure straight away. A 3 × 3 metre slab at 100 mm thick is 0.90 m³ before waste. Add 8% and you order 1.0 m³ because ready-mix lorries deliver in 0.25 m³ increments. Bag yields on UK pre-mixed concrete: a 20 kg bag gives about 0.010 m³, a 25 kg bag about 0.0125 m³.

Reinforcement estimates assume 12 mm bar at 200 mm centres each way — the typical residential slab to BS 8500. Wire mesh on a domestic slab is usually A142 (for paths) or A193 (for driveways) per BS 4483.

Australia works in cubic metres — length × width × thickness in metres gives the order figure directly. A 3 × 3 metre slab at 100 mm is 0.90 m³ before waste. Add 8% and order 1.0 m³; batching plants deliver in 0.2 m³ increments via agitator trucks. Bag yields from Boral, Hanson, and Cement Australia: a 20 kg bag gives about 0.010 m³, a 30 kg bag about 0.015 m³.

Reinforcement estimates use N12 deformed bar at 300 mm centres each way — typical for residential exposure class A1 or A2 to AS 3600. For waffle slabs and stiffened rafts under AS 2870, the bar schedule is engineered to site reactivity class.

Canada specifies in cubic metres — length × width × thickness in metres gives the order figure. A 3 × 3 metre slab at 100 mm thick is 0.90 m³ before waste. Add 8% and you order 1.0 m³; ready-mix is delivered in 0.25 m³ increments. Canadian bag yields (Sika, Quikrete Canada, King Packaged Materials): a 25 kg bag gives about 0.0125 m³, a 30 kg bag about 0.015 m³.

Reinforcement estimates use 15M bar at 300 mm centres each way — common for a residential slab or driveway under CSA A23.1. For unheated exterior slabs, exposure class C-2 or F-1 also drives the cover and air-entrainment requirements.

UK building regs don't prescribe a slab-on-ground thickness directly for non-habitable hard surfaces, but the NHBC and Concrete Centre guidance points to 100 mm minimum for domestic driveways and 150 mm for any slab carrying delivery vehicles. Reactive clay sites need engineered foundations regardless.

For most Australian houses, the slab is designed under AS 2870 with site classification by soil reactivity — Class A through P. Reactive clay (Class H, E, P) is common across Adelaide, Melbourne, Sydney's south-west, and SE Queensland, and demands engineered waffle pods or stiffened rafts with 600–800 mm deep edge beams.

Foundation depth is set by frost across most of Canada — 1.2 m in southern Ontario and the Lower Mainland, 1.5–1.8 m across the Prairies and Quebec, up to 2.4 m in the North. Slabs-on-grade in exterior settings should be air-entrained with C or F exposure class per CSA A23.1.

UK concrete is specified by strength class — C25/30 means 25 N/mm² cylinder strength and 30 N/mm² cube strength at 28 days. For residential work, designated mixes (named codes like ST2, GEN1, RC25/30) are easier than designed mixes — the supplier knows what's in them.

The exposure class is the part that drives durability. XC covers carbonation, XF covers freeze-thaw, XD and XS cover chlorides. Cover requirements per BS 8500 Table A.5: 25–50 mm depending on class. Most domestic foundations sit at XC2 with 40 mm nominal cover.

Australian concrete grades use the N prefix (Normal class) with the characteristic strength in MPa: N20, N25, N32, N40, N50. Special class (S) covers performance-specified mixes — used for B2, C1, C2, and U exposure under AS 1379.

Exposure class drives minimum grade and cover under AS 3600. Within 1 km of surf or 100 m of still saltwater puts you in B1 or B2 — N32 minimum with 40 mm cover. Tidal and splash zones (C1, C2) need N50 or designed special-class mixes.

Canadian concrete is specified in MPa at 28 days, plus an exposure class that drives air entrainment, water-cement ratio, and minimum cement content. Classes come from CSA A23.1:24.

If a driveway sees road salt, specify C-2 minimum — N and F-class won't last. C-class covers chlorides, F-class covers freeze-thaw with low chlorides, N is interior or sheltered work only. For Canadian exterior slabs, never order without air entrainment.

For cold-weather pours, keep concrete temperature above 5°C until it reaches around 5 N/mm². Below freezing, that means heated water, accelerator admixtures (chloride-free for reinforced work), and insulating quilts. Never pour on frozen ground — strip topsoil and warm the base first.

UK concrete is specified by consistence class per BS EN 206: S1 (10–40 mm slump) is stiff, S3 (100–150 mm) is the typical residential pour, S4 (160–210 mm) is pumpable. Higher classes should be achieved with plasticisers rather than added water.

Australian summers reverse the problem — heat is the enemy. Above 32°C, surface evaporation outpaces bleed water and plastic shrinkage cracking starts within minutes. Pour early morning, shade with sails or hessian within minutes of finishing, and apply evaporative retarder before screeding. AS 1379 caps delivery temperature at 35°C for normal-class concrete.

Slump is specified at the truck per AS 1012.3.1, typically 80 mm for residential flatwork and 100–120 mm for pump work. Curing membrane on for 7 days minimum, longer in WA and NT summers. Cover requirements jump with exposure class — coastal sites need 40–50 mm cover under AS 3600.

Cold-weather concreting is normal practice across Canada. CSA A23.1 defines cold conditions as below 5°C, and requires the concrete stay above 10°C until it reaches at least 5 MPa. That means heated water and aggregates, Type HE cement or an accelerator, insulated tarps or hoarding with propane heaters, and continuous monitoring for the first 72 hours. Concrete that freezes before 5 MPa loses long-term strength permanently.

Slump is specified in mm per CSA A23.2-5C — typically 80–100 mm for residential flatwork. Air content for exterior slabs runs 4–8% depending on exposure class; your supplier verifies at the truck. Never pour on frozen subgrade — thaw it with ground heaters first.